In this chapter you
will learn:
• to integrate using
known derivatives
• to use the chain rule
in reverse
19
• to integrate using
trigonometric identities
• to integrate using
inverse trigonometric
functions
• to use the product rule
in reverse (integration
by parts)
• to integrate using a
change of variable
(substitution)
• to integrate using
the separation of
a fraction into two
fractions.
Further
integration
methods
Introductory problem
Use integration to prove that the area of a circle of radius r
is equal to πr2.
Having extended the range of functions we can differentiate,
we now need to do the same for integration. Sometimes we
will be able to use results from the previous chapter, but in
other cases we will require new techniques. In this chapter we
look at each of these in turn and then face the challenge of
selecting the appropriate technique from the list of options
we have.
19A Reversing standard derivatives
In chapter 17 we reversed a number of standard derivatives
that had been established in chapter 16 to give us this list of
functions we could integrate.
t
exam hin
ll
These are a
e
th
in
given
klet.
Formula boo
622 Topic 6: Calculus
∫x
∫e
n
dxx
x
dx
1 n
x + c,
n +1
x
+c
≠ 1
1
∫ x dx = l x + c
∫ sin x dx = − cos x + c
si x + c
∫ cos x dx = sin
© Cambridge University Press 2012
In chapter 18 (Key point 18.3) we differentiated sec x, csc x and
cot x. We can now reverse these standard derivatives too and
add them to our list:
∫ sec x dx = ta x + c
∫ sec x ttann x dx = sec x + c
∫ csc x cot x dx csc x + c
∫ csc x dx = −cot x + c
exam hint
ot given in
These are n
ndard
the list of sta
the
integrals in
let, but
k
o
o
Formula b
uced from
can be ded
ndard
the list of sta
derivatives.
The chain rule for differentiation (chapter 18A) allows us to go
further and deal with integrals such as ∫ 2cos(2x )dx . Here we
think about integrating cos to sin and then consider what the
chain rule would give us if we differentiated back. In this case
d
the chain rule would give the 2 anyway as (sin2 ) 2 cos22x
dx
(2 is the derivative of 2x) so we have the correct integration
straight away:
∫ 2cos (
) dx = sin
sin
i 2x + c
We may have a similar question in which we do not have the
exact derivative and then we need to compensate by cancelling
out any unwanted constant generated by the chain rule.
For example, in finding ∫ (
)4 dx we proceed as before
1
5
chain rule gives us an unwanted 2:
d ⎛1
(2 x 3)5 ⎞ = 2 (2x
2 x 3 )4
⎠
dx ⎝ 5
integrating (
)4 to ( )5 but now when we differentiate back the
so we divide by 2 to remove it:
1 1
1
5
× ( − ) + c = ( − ) + c.
2 5
10
You may notice a pattern here, we always divide by the
coefficient of x. This is indeed a general rule, which follows
simply by reversing the special case of the chain rule from
Key point 18.2.
∫(
−
)4 d x =
KEY POINT 19.1
The reverse chain rule
∫f
where
1
ax
a + b)dx = F (ax + b ) + c
a
( ) is the integral of
f x ).
© Cambridge University Press 2012
exam hint
ly applies
This rule on
nside’
when the ‘i
f the form
o
function is
(ax + b)!
19 Further integration methods
623
With this shortcut we do not need to work through the chain
rule every time.
Worked example 19.1
Find the following:
1
(a) ∫ e 4 dx
2
(b)
2
∫ 5 − x dx
Integrate e( ) to e( ) and divide by
the coefficient of x
1
∫2e
(a)
1
()
to ln
and divide
dx =
1 1 4x
× e +c
2 4
1 4x
e +c
8
=
Integrate
4
⎛ 1 ⎞
2
∫ 5 − x dx = 2 ⎝ −1 ⎠
(b)
= −2
by the coefficient of x
5−x +c
5−x +c
Exercise 19A
1. Find:
(a) (i)
∫ 5 ( x 3)
(b) (i)
∫ ( 4 x 5 ) dx
∫ 4 ( 3 − x ) dx
(c) (i)
4
dx
7
6
1
2
(d) (i)
∫
(e) (i)
∫
2 x − 1 dx
1
4
2+
dx
x
3
(ii)
(ii)
∫ ( x ) dx
∫ ( x + 1) dx
5
3
1
8
(ii)
∫(
)8 dx
(ii)
∫7 (
−
(ii)
∫(
6
−
−
)3/ 4 dx
)2
dx
2. Find these integrals:
(a) (i)
∫ 3e
(b) (i)
∫ 4e
(c) (i)
∫ −6e
(d) (i)
624 Topic 6: Calculus
3
dx
2x 1
3
(ii)
∫e
(ii)
∫ e dx
(ii)
∫e
(ii)
∫e
dx
1
dx
−3
−2
∫ e x / 4 dx
dx
x
1
4x
dx
2
− x
dx
© Cambridge University Press 2012
3. Find:
1
(a) (i)
∫ x + 4 dx
(b) (i)
∫3
(c) (i)
∫ 1 − 4 x dx
(d) (i)
∫ 1 − 5 − x dx
2
4
(ii)
∫5
5
2
dx
−8
dx
−5
1
(ii) ∫
dx
7 2x
1
(ii) ∫ 3 +
dx
3−x
dx
(ii)
−3
3
∫2
4. Integrate the following:
(a)
(b)
(c)
(d)
(e)
(f)
∫ − csc x cot x dx
∫ 3 sec 3 x dx
) dx
∫ sin (
⎛1 ⎞
∫ csc ⎝ 4 ⎠ dx
∫ 2 cos 4x dx
x
x
∫ sec 2 tan 2 dx
2
1
5. Two students integrate ∫ dx in two different ways.
3x
Marina writes:
1
1 1
1
∫ 3x dx 3 ∫ x dx 3 ln x + c
Jack uses the special case of the reverse chain rule and
divides by the coefficient of x:
1
∫ 3 x dx
1
l 3x
3x + c
3
Who has the right answer?
6. Given that 0
1 and the area between the x-axis, the
1
lines x a2, x a and the graph of y =
is 0.4, find the
1− x
value of a correct to 3 significant figures.
[5 marks]
© Cambridge University Press 2012
19 Further integration methods
625
19B Integration by substitution
Another method for
integrating products
is integration by
parts, the reverse
of the product rule.
We will meet this in
Section 19F.
The shortcut for reversing the chain rule works only when the
derivative of the ‘inside’ function is a constant. This is because a
constant factor can ‘move through the integral sign’, for example:
1
1
1
∫ cos2 d ∫ 2 × 2 cos 2x dx 2 ∫ cos d 2 sin 2x + c
This cannot be done with a variable: ∫ x sin x dx is not the
same as x ∫ sin x dx . So we need a different rule for integrating
a product of two functions. In some cases this can be done by
extending the principle of reversing the chain rule, leading to
the method of integration by substitution.
When using the chain rule to differentiate a composite function,
we differentiate the outer function and multiply this by the
derivative of the inner function; for example
d
cos((x
(x 2
) × 2x
(sin (x 2 )) cos(x
dx
We can think of this as using a substitution u = x 2 + 2, and then
dy dy du
=
× .
dx du dx
Let us now look at ∫ x cos( x 2 + 2) dx.
626 Topic 6: Calculus
As cos( x 2 + 2) is a composite function we can write it as cosu,
where u = x 2 + 2. So our integral becomes ∫ x s u dx. We know
how to integrate cosu, so we want to change our variable to
u. But then we need to be integrating with respect to u, so we
should have du instead of dx. Those two are not the same thing,
du
but they are related because u = x 2 + 2 ⇒
= 2x .
dx
1
We can now ‘rearrange’ this to get d
du .
2x
Substituting all this into our integral we now have
⎛ 1⎞
os u ⎜ ⎟ du
∫ x cos ( 2 + 2) ∫ x ccos
⎝ 2x ⎠
1
= ∫ cos u du
2
1
= sin
si u + c
2
This answer is in terms of u so we need to write it in terms of x.
1
dx
s (x 2 + 2) + c
∫ x cos(x 2 + 2) dx
2
du
A word of warning here:
is not really a fraction, so it is not
dx
clear that the above ‘rearrangement’ is valid. However, it can
be shown that it follows from the chain rule that it is valid to
1
replace dx by
du.
f ′( )
© Cambridge University Press 2012
Worked example 19.2
Find the following:
(a) ∫ sin5 x ccos x dx
(b) ∫ x 2
x3 4
It is helpful here to think of sin5 x
5
as ( sinx ) . Therefore the inner
function is sin x
Make the substitution
dx
(a) Let u = sin x
du
Then
=
x⇒d
dx
∫
∫
5
1
du
cos x
cos
o x
1
du
cos x
= ∫u 5 du
Write the answer in terms of x
ex
3+4
is a composite function with
inner function x 3 + 4
Make the substitution
1 6
u +c
6
=
1 6
sin x + c
6
(b) Let u = x 3 + 4
du
Then
= 3x2 ⇒ d
dx
∫x e
2 x3
∫x e
2
d
=∫
Write the answer in terms of x
=
1
du
3x2
1
e du
3
=
1 u
e +c
3
=
1 x3 + 4
e
+c
3
You may have noticed in all of the above examples that, after
making the substitution, the part of the integrand which was
du
still in terms of x cancelled with a similar term coming from .
dx
1
1
3
For example, in (b), ∫ x 2 x 4 dx ∫ x 2 eu 2 du ∫ u du .
3x
3
This will always happen when one part of the integrand is an
exact multiple of the derivative of the inner function, and can be
explained by looking at the chain rule.
© Cambridge University Press 2012
1
du
3x2
exam hint
nd is the
The integra
to be
expression
integrated.
19 Further integration methods
627
For example, consider the integral
∫ ( 2 x 3) ( x
2
+ 3 x 5 ) dx .
4
To find this integral, think about what we would need to
4
differentiate to get (2 3) ( x 2 3x 5) . As 2 3 is the
derivative of x 2 + 3x
3x 5 we know that we would get 2 3 ‘for
3x 5 using the
free’ when differentiating some power of x 2 + 3x
4
3x 5) we would
chain rule. In this case to end up with ( x 2 + 3x
1
5
want to be differentiating ( x 2 + 3x 5) , that is
5
d ⎛1 2
5⎞
4
3x 5) ⎟ = (2x
2 x 3) ( x 2 + 3x
3 x 5)
⎜⎝ ( x + 3x
⎠
dx 5
and therefore:
1 2
( x + 3x 5)5 + c
5
This is the same answer we would get by using the substitution
u = x 2 + 3x − 5. If you notice that you can integrate an
expression by reversing the chain rule, you can just write down
the answer without any working. However, if you are not sure, it
is safer to go through the whole process of substitution.
∫ (2
3) ( x 2
3 x 5 ) dx
4
In some cases this cancelling of the remaining x-terms will not
happen and you will have to express x in terms of u. The full
method of substitution will then be as follows:
KEY POINT 19.2
t
exam hin
rly
You will nea which
ld
to
e
b
always
use. If
to
n
o
substituti
look for
you are not,
function
a composite
= ‘inner’
and take u
function.
Integration by substitution
1. Select a substitution (if not already given).
2. Differentiate the substitution and write dx in terms
of du.
3. Replace dx by the above expression, and replace any
obvious occurrences of u.
4. Simplify as far as possible.
5. If any terms with x remain, write them in terms of u.
6. Work out the new integral in terms of u.
7. Write the answer in terms of x.
For the integral in the next example there are two possible
substitutions. As there is a composite function 4 x 1, we
could use the ‘inner’ function: u 4 x − 1. However, we must
always use the substitution we are given.
628 Topic 6: Calculus
© Cambridge University Press 2012
Worked example 19.3
Find ∫ x 4 x 1 dx using the substitution u
Differentiate the substitution
4 x − 1.
u = 4x − 1
du
4
⇒
=
dx 2 4 x − 1
=
... and write dx in terms of du
(Key point 19.2 Step 2)
Replace those parts that we
already have expressions for,
and simplify if possible
(Steps 3 and 4)
There is still an x remaining,
so replace it by using
u2 + 1
u
4x − 1 ⇒ x =
4
(Step 5)
∴d =
∫x
4x
2
u
1
du
2
1
∫ xu 2 u du
1d
1
= ∫ xu 2 du
2
=∫
1 u2 + 1 2
u du
2 4
Now everything is in terms of
u so we can integrate (Step 6)
=
1⎛1
1 ⎞
1 4
u + u 2 du = ⎝ u 5 + u 3 ⎠ + c
∫
8 5
3
8
Write the answer in terms of x
=
1⎛1
8⎝5
using u
4x − 1 (Step 7)
(
−
)
5
+
1
3
(
−
) ⎞⎠ + c
3
When limits are given, we must change them too. Then there is
no need to change back to the original variable at the end.
KEY POINT 19.3
When evaluating a definite integral using substitution, add
the following step to the process in Key point 19.2:
Step 3a. Write the limits in terms of u.
© Cambridge University Press 2012
19 Further integration methods
629
The next example shows one of the most common uses of
substitution; integrating a quotient where the numerator is a
multiple of the derivative of the denominator.
Worked example 19.4
Evaluate ∫
1
0
x −3
dx giving your answer in the form a ln p.
x 6x + 7
2
This is of the form ‘something’ ×
1
( )
so the ‘inner’ function is x 2 6x
6
7
Write limits in terms of u
Make the substitution
Simplify 2
6 2(
3)
Let u = x 2 − 6 x + 7.
Then
du
1
= 2x 6 ⇒ d =
du
dx
2x − 6
Limits: x = 0 ⇒ u = 7, x = 1 ⇒ u = 2
∫
1
0
x 3
x−3 1
x=∫
du
7
x 6x + 7
u 2x − 6
=∫
2
7
1
du
2u
2
⎡ 1
⎤
= ⎢ ln u ⎥
⎣2
⎦7
1
( −
2
1
2
= ln ⎛ ⎞
2 ⎝ 7⎠
=
)
In the above example it is possible to write down the result of
the integration without using the full substitution method, if we
6 x 7,
notice that x − 3 is half of the derivative of x 2 6x
1
and so ( x − ) × 2
comes from differentiating
x 6x + 7
1
ln x 2 6 x 7 .
2
630 Topic 6: Calculus
© Cambridge University Press 2012
This particular case of substitution, where the top of the fraction
is the derivative of the bottom, is definitely worth remembering:
KEY POINT 19.4
∫
f ′ x)
dx
f x)
f (x) +c
We will see more
examples of using
trigonometric identities in the next
section.
The next example shows that the substitution can also be
given as x in terms of u (or θ in this case). It also illustrates
that substitutions can lead to integrals where the use of
trigonometric identities is required.
Worked example 19.5
2
−32
Use the substitution x = secθ to find the exact value of ∫ ( x 2 1) ( ) dx.
2
Differentiate the substitution and
express dx in terms of dθ
x = secθ
⇒
dx
= sec θ tan
ta θ
dθ
θ
d
Change the limits
θ dθ
When x = 2 :
secθ
1
2
⇒ cos
co θ =
⇒θ =
π
4
When x = 2 :
secθ
⇒ co θ =
⇒θ =
© Cambridge University Press 2012
π
3
1
2
19 Further integration methods
631
continued . . .
Replace those parts that we
already have expressions for.
Remember that sec2θ 1 tanθ
2
∫ (x
2
=∫
π3
=∫
π3
=∫
π3
π4
π4
π4
We seem to be stuck; writing
everything in terms of sin and cos
often helps
1 cosθ
, we
sinθ sinθ
now have a standard derivative
(Section 19A)
Writing this as
− 1)
2
−
3
2
3
(
π3
∫ (
x
)− 2
1)
2
π4
−
3
2
secθ tan
ta θ dθ
seccθ tanθ dθ
(
)−3 seccθ tanθ dθ
(
)−2 seccθ dθ
1 cos2 θ
dθ
π 4 cos θ sin2 θ
π 3 cos θ
=∫
dθ
π 4 sin2 θ
=∫
π3
=∫
π3
π4
cscθ cot θ dθ
⎛
] = −
π3
= [−
⎝
π4
2 ⎞
⎟− −
3⎠
(
)=
2−
2
3
Exercise 19B
1. Either by using a suitable substitution, or by considering the
chain rule, find these integrals:
∫ x ( x + 3 ) dx
(b) (i) ∫ (2 x 5) (3x 15x 4 )
(ii) ∫ ( x 2xx ) ( x 3x − 5)
2x
(c) (i) ∫
dx
x +3
(d) (i) ∫ 4 s 3x sin
sin 3x dx
(e) (i) ∫ 3x
dx
e
(f) (i) ∫
dx
e
+4
3
2
(a) (i)
2
2
5
3 x2 1
2
2
632 Topic 6: Calculus
3
3
(ii)
2
4
3
3
2
∫3 (
2
1) dx
5
dx
dx
6 2 12
∫ x 3 6 x 1 dx
(ii) ∫ cos 2 x sin
si 3 2 x dx
(ii)
∫ 3 x dx
cos x
(ii) ∫
dx
3 4 sin x
(ii)
x2
© Cambridge University Press 2012
(g) (i)
∫ 32 sec
2
2 tan
ta 3 2 x dx
x
x
(ii) ∫ 6 sec6 ⎛ ⎞ tan ⎛ ⎞ dx
⎝ 4⎠
⎝ 4⎠
(h) (i)
∫ csc
4
x cot
c t x dx
csc2 2 x
∫ (3 cot 2x ) dx
x
(i) (i) ∫
dx
3 − x2
(ii) ∫ 2e −4 x +1 e
dx
(ii)
2. Use a suitable substitution to show that ∫ tan x x = ln sec x + c.
3. Find the following integrals using the given substitution:
(a) (i)
(ii)
(b) (i)
(ii)
∫x
∫x
x + 1 dx
dx, u = x + 1
2
x − 2 dx
dx , u
x 2
exam hint
l of tan
The integra
in the
n
is not give
klet,
o
Formula bo
h
and is wort
g.
n
ri
e
b
remem
∫ 2 ( 5 ) dx , u = x − 5
∫ x ( x + ) dx , u = x + 3
7
5
4. Find the following using an appropriate substitution:
(a) (i)
∫ x (2x 1)
(b) (i)
∫x
(c) (i)
4
dx
x − 3 dx
x2
dx
x −5
∫
(ii)
∫ 9 (3 x + 2 )
(ii)
∫ (x + )
(ii)
∫ (2
5
4(
dx
5 x − 6 dx
5)
3)
3
dx
5. Use the given substitution to evaluate these definite integrals:
(a) (i)
(ii)
(b) (i)
(ii)
(c) (i)
(ii)
2
⎛ x ⎞
∫2 ⎝ 4 − x ⎠ dx, u = 4 − x
3
x3
∫1 ( x + 2)2 dx, u = x + 2
π 6 cos θ
∫0 1 + sinθ dθ , sinθ
π 2 sin θ
cosθ
∫0 1 + cosθ dθ ,
1/ 3
1
2
∫0 4 9x 2 dx, x = 3 sin θ
3
∫
14
0
t
exam hin
nusual
When an u
is
substitution
ill
w
it
required
en in
iv
g
e
always b
.
n
the questio
1
1
dx , x = cosθ
2
2
1 4x
© Cambridge University Press 2012
19 Further integration methods
633
2
6. Find the exact value of ∫ (2 x 1)
x 2 + x −1
0
7. Evaluate ∫
5
2
dx .
2x
dx , giving your answer in the
x2 − 1
form ln k.
[4 marks]
8. Use the substitution u
x − 2 to find ∫
) is a factor of x 3 − 1.
9. (a) Show that (
2x 2 x 1
(b) Find ∫
dx .
x3 − 1
10. Use the substitution u
11. Find ∫
ln x to find ∫
cos x
dx .
sin 5 x
12. Evaluate ∫
[6 marks]
3
1
(2 x
x
dx .
x −2
[6 marks]
[4 marks]
sec ( ln(
ln(x 2 ))
dx. [6 marks]
2x
[3 marks]
3) x 2 − 3
x 2 3x 3
3
dx .
[6 marks]
19C Using trigonometric identities in
integration
t
exam hin
sked to
If you are a
ral like
do an integ
xam you
e
this in the
n a hint,
will be give
ample
as in the ex
below.
634 Topic 6: Calculus
Sometimes it is necessary to rearrange the expression before
reversing a standard derivative or using a substitution. In
this section we will take a more systematic look at using
trigonometric identities in order to integrate a wide range of
functions.
As seen in the previous section, the presence of the cos x in
∫ sin
3
x cos
c x dx makes it possible to apply the reverse chain rule
(or a substitution) but how do we cope with just ∫ sin3 d ? As
a mixture of sin and cos helps us in the use of the reverse chain
rule, we aim to introduce cos by using sin2 x cos2 x = 1.
© Cambridge University Press 2012
Worked example 19.6
(a) Show that sin3 x
sin x − cos2 x sin x.
(b) Hence find ∫ sin3 x dx .
(a) sin3 x
Introduce cos2 x by using sin2
cos2 x = 1;
to do this we need to ‘split’ sin3 x
=(
i
sin
i x
2
) sin x
= sin x − ccos
os2 x sin x
We can use the result from part (a)
(b)
Use a substitution u
u=
∫ sin
3
x dx
∫ sin x
cos2 sin
co
i dx
du
= − sin x
dx
1 ⎞
⎛
∫ sin3 x dx = − cos x − ∫ u 2 sin x ⎝ sin x ⎠ du
cos x
⇒
= − cos x + ∫ u 2 du
= − cos x +
The same trick does not work for ∫ sin2 x dx , as we can only
rewrite it as ∫ 1 − cos2 x dx which we also cannot integrate.
Instead we notice that sin2 x appears in one of the versions of
in2 x, and
the double-angle formulae for cos 2 : cos 2 x 1 2 sin
we know how to integrate cos2x.
1
ccos
os3 x + c
3
Double angle identities were covered in
Section 12A.
Worked example 19.7
Find sin2 x dx.
Write an alternative expression for
sin2 x by using a double angle identity
cos 2 x
⇒ in2
∴∫
Remember to divide by the coefficient
of x when integrating cos 2x
© Cambridge University Press 2012
1 2 sin
i 2x
=
1
(1 − cos
2
)
1
(1 − cos 2x ) dx
2
1 1
= ∫ − cos 2 x dx
2 2
=∫
1
1 1
x−
i 2x + c
2
22
1
1
= x−
i 2x + c
2
4
=
19 Further integration methods
635
A similar method is used to integrate cos2 x and should be
learnt.
exam hint
expected
You will be
method
to recall this
.
without hints
KEY POINT 19.5
To integrate sin2 x , use cos 2 x 1 2 sin
in2 x.
To integrate cos2 x, use cos 2 x
2 cos2 x − 1.
The methods from Worked examples 19.6 and 19.7 can be
extended to deal with any powers of sin x and cos . The
method from Worked example 19.6 can be applied to any odd
power, for example:
sin5
(sin2 x )2 sin
(1 cos2 x )2 sin
c 2 x + cos 4 ) sin
in x
(1 2 cos
which can be integrated using the reverse chain rule.
For even powers, we can use the identity from Key point 19.5,
for example
1 cos 2 x ⎞ 2 1 1
1
2
⎛
4
2
cos
= + cos 2 x + cos2 2 x
(cos x ) = ⎝
⎠
2
4 2
4
and then the double angle identity has to be used again to relate
cos2 2x to cos 4x.
See Worked example 15.25 in Section
15H to remind you
of this method.
This becomes increasingly complicated for larger powers.
Luckily, there is a ready-made alternative from the unlikely
source of complex numbers. We saw in chapter 15 that we could
use De Moivre’s Theorem to generate expressions for powers of
sin and cos in terms of multiple angles.
Worked example 19.8
1
3
15
5
cos66x
6 x + cos 4 x + cos2
cos 2x
2x + .
16
8
16
8
(b) Hence find ∫ cos6 x dx.
(a) Show that cos6 x =
We derived similar identities in
Section 15H, so we will not
repeat it here
Use the result from part (a)
Don’t forget to divide by the
coefficient of x
636 Topic 6: Calculus
(a) See Section 15H for how to do this.
(b) ∫ cos6 x dx
=
1
cos 6 x
∫ 16 cos6x
3
cos 4 x
8
15
cos 2x
2x
16
5
dx
8
1
3
15
5
sin6
6x +
sin
i 4x +
sin 2 x + x + c
96
32
32
8
© Cambridge University Press 2012
Don’t worry if this seems complicated – a question like this will
always be split into several parts, as in questions 10 and 11 in
Exercise 19C at the end of this section.
We shall now integrate tan x and its powers. We have already
integrated tan x as an application of the reverse chain rule in
Exercise 19B, question 2: ∫ tan x dx
lln sec x
c. However,
This identity was
derived in chapter 12.
this does not help when trying to integrate more complicated
functions, for example tan2 x. We do, however, have an identity
relating tan2 x to something we know how to integrate:
1 + ta 2 x = sec2 x .
Worked example 19.9
Find ∫ tan2 2x dx .
We have an identity relating
tan2 ( ) to sec2 ( ), which we
know how to integrate
Use the standard result for
integrating sec2 ( ), remembering
to divide by the coefficient of x
∫ tan
2
=
d
∫ sec
2
2 x 1 dx
1
tan 2 x − x + c
2
The same identity is used in integrating any power
of tan x.
KEY POINT 19.6
To integrate tann x use the identity 1 + tan2 x = sec2 x and
d
the fact that
( tan x ) sec2 x.
dx
© Cambridge University Press 2012
19 Further integration methods
637
Worked example 19.10
Find ∫ tan3 d .
∫ tan
Introduce sec2 x by using
tan2
sec2 x − 1
3
We found ∫ tan x in the previous
section
∫ a
= ∫(
tan x dx
2
) tan x dx
= ∫ sec2 tan
ta x dx − ∫ tan
ta x dx
We integrate the two terms separately
We can apply the reverse chain
rule (or a substitution u tanx ) to
d
sec2 tanx because
(tan x ) = sec 2 x
dx
1 2
( ) integrates to 2 ( )
d
First integral:
∫ sec
2
tan
ta x =
Second integral:
∫ tan x dx
∴ ∫ tan3 d
1
( tan x )2 + c
2
ln sec x + c
1
tan2 x − ln
l sec
ec x + c
2
The above examples illustrate standard methods used to
integrate powers of trigonometric functions. Many other
trigonometric integrals can be rearranged into a form where
we can simply reverse a standard derivative. Here we give one
example of using trigonometric identities to do this.
Worked example 19.11
Find ∫
sin 4 x
dx .
sin 3 2 x
As we have 4x and 2x, apply the double
angle identity for sine
638 Topic 6: Calculus
sin 4 x
dx
3
2x
∫ sin
2sin
2
ssinn 2 x cos
co 2 x
dx
3
sin 2 x
2 cos 2 x
=∫
dx
sin2 2 x
∫
© Cambridge University Press 2012
continued . . .
Check if this can be manipulated into the form
of one of our standard derivatives. If we
cannot see it immediately, it is a good idea to
try and split the expression into a product of
two trigonometric functions
Remember to divide by the coefficient of x
= 2∫
1 cos 2 x
dx
sin 2 x sin 2 x
= 2∫ csc 2 x cot
cot 2 x dx
= − csc2x + c
Exercise 19C
1. Simplify to get standard integrals, and then integrate:
tan 3x
(a)
∫ cos 3x dx
(b)
∫ sin
(c)
∫ sin 5
1
2
x
dx
cos 5 sin x dx
cos
2x
dx
sin2 2 x
cos 2x
(e)
dx
∫ cos x + sin
in x
(d)
∫
3
2. Use trigonometric identities before using a substitution (or
reversing the chain rule) to integrate:
(a)
∫ cos
3
x sin2 x dx
cos3 x
∫ sin2 x dx
(c) ∫ sin cos xecos
(b)
(d)
∫ tan
(e)
∫
4
x
dx
tan
a 6 3 x dx
3
sin 2 x cos2
c 2x
cos
dx
1 cos 4 x
3. Find the following integrals:
(a) (i)
∫ 2 cos
2
x dx
(ii)
∫ cos
(b) (i)
∫ 2 tan
2
⎛ x⎞
⎜⎝ ⎟⎠ dx
2
(ii)
∫ tan
© Cambridge University Press 2012
2
3x dx
2
3x dx
19 Further integration methods
639
4. Find the exact value of the following:
π
(a) (i)
∫
(b) (i)
∫ ( tan x )
0
sin2 2x dx
π/4
(ii)
2
0
dx
(ii)
∫
2π
0
x
tan2 ⎛ ⎞ dx
⎝ 6⎠
π
∫ (1
π/4
2
)2 dx
5. Three students integrate cos x sin x in three different ways:
Amara uses reverse chain rule with u sin x :
du
= cos x , so
dx
∫ cos x sin x dx ∫ u du
1
= si 2 x + c
2
Ben uses reverse chain rule with u
x:
du
= −sin x , so
dx
∫ cos sin x dx ∫ u du
1
= − cos2 x + c
2
Carlos uses a double angle formula:
1
∫ cos x sin x dx ∫ 2 sin x dx
1
= − cos 2 x + c
4
Who is right?
x
6. Find ∫ sin2 ⎛ ⎞ dx.
⎝ 3⎠
7. (a) Show that tan3 x
[5 marks]
t n x sec2 x tan
tan
t n x.
(b) Hence find ∫ tan3 x dx .
π /12
8. Given that ∫
0
value of k.
tan (kx ) dx =
[6 marks]
4−π
find the
12
9. (a) Use the formula for cos( A B) to show that cos2 x
(b) Hence find ∫ cos 2x sin x dx.
640 Topic 6: Calculus
[6 marks]
2 cos2 x − 1
[7 marks]
© Cambridge University Press 2012
10. (a) Show that sin3 θ
sinθ − sinθ cos2 θ .
3π
x
(b) Hence find the exact value of ∫ sin3 ⎛ ⎞ d . [7 marks]
0
⎝ 3⎠
11. A complex number is defined by z = cosθ + sinθ .
1
(a) (i) Show that = cosθ − sinθ .
z
(ii) Use De Moivre’s Theorem to deduce that:
1
z n − n = 2isin nθ .
z
1⎞ 5
⎛
(b) (i) Expand z −
.
⎝
z⎠
(ii) Hence find integers a, b and c such that:
16 sin5 θ
si 5θ b in33θ
sin
sinθ .
(c) Find ∫ sin 2 d .
5
[14 marks]
19D Integration using inverse
trigonometric functions
In the last section we saw examples of similar-looking integrals
that required very different methods: ∫ sin 2 cos x dx could
be done by reversing the chain rule. But ∫ sin2 x dx required
the more complex method of substituting with a trigonometric
identity; without the derivative of sin x the integration was more
difficult.
x
1
dx and ∫
dx .
Similarly, consider ∫
2
1− x
1 − x2
The first integral features a function (
) and a multiple of
), so we can apply the reverse chain rule:
its derivative (
x
1 − x 2 + c. However, the absence of the
∫ 1 − x 2 dx
) in the second integral means that we need
derivative of (
another method.
Fortunately, we have already met the expression
1
in
1 − x2
chapter 18 as the derivative of arcsin x. In the same chapter we
1
saw that the derivative of arctan x is
. This means that:
1 + x2
∫
1
1 − x2
=
1
∫ 1+ x
© Cambridge University Press 2012
2
These results are
given in Key point
18.8.
dx = r t n x .
19 Further integration methods
641
We can extend these results slightly to include expressions of the
1
form 2
and 1 . For example, to integrate 1
a x2
a2 x 2
16 + x 2
1
we can take out a factor of
to turn the denominator into the
16
form 1 + Y 2 and then use a substitution:
1
∫ 16 + x
2
=∫
1
dx
x2
16 (1 + 16
)
1
1
x
× 4dY where Y =
∫
2
16 1 + Y
4
1
1
= ∫
dY
4 1+Y2
1
= arctanY + c
4
1
⎛ x⎞
= arctan
+c
⎝ 4⎠
4
=
We can use this method to obtain the general result for the two
integrals:
KEY POINT 19.7
∫a
∫
You may wonder
whether there are rules
for integrating
1
when x > a or
2
x
a2
1
. These require the
x 2 a2
study of hyperbolic functions,
which are in many ways
similar to trigonometric
functions, and can be used
to describe some important
curves, such as the shape of
a hanging chain.
642 Topic 6: Calculus
1
2
1
x
dx = arctan ⎛ ⎞ + c
⎝ a⎠
x
a
2
1
a
2
⎛ x⎞
dx = arcsin ⎜ ⎟ + c ( x < a)
⎝ a⎠
x
2
It is worth noting several things about these; first of all, the
1
arctan result has a factor of and the arcsin one does not.
a
Secondly, the arcsin result only applies when x < a because of
the presence of the square root. Finally, you may be wondering
why there is no corresponding result with arccos; if you look
back at Key point 18.7 you will see that the derivative of
1
arccos x is −
, which, according to the Key point above,
1 − x2
would integrate to −arcsin x. This is because the graphs of the
arcin and the arccos functions are related through a reflection
and a translation.
© Cambridge University Press 2012
Worked example 19.12
Find the value of: ∫
1
dx
1 9x 2
This is similar to the derivative of arctan x
1
9x2
∫1
(a)
but x2 has been replaced by 9 2 (3x )
So reverse the standard derivative,
remembering to divide by the
coefficient of x
2
=∫
=
1
1
(3x )2
dx
1
arctan(3 ) + c
3
When working through an integration, we may first need to put the expression into the correct
form. As there is an x2 term in the derivatives of both arcsin x and arctan x, this often involves
completing the square. (For a reminder of this term, see glossary on CD-ROM.)
Worked example 19.13
Find ∫
3
−44 x
2
4x + 8
dx .
This is not a reverse chain rule integral
and there is a square root in the
denominator, so perhaps arcsin? The
only way of producing 1− X 2 in the
denominator is to start by completing
the square to get C
X2
Now reverse the standard derivative,
remembering to divide by the
coefficient of x which is 2
3
∫
−4
4x
=∫
=∫
= 3∫
=
2
=∫
4x + 8
3
− ⎡⎣( 2 x + 1) − 1 − 8 ⎤⎦
2
3
9 − ( 2 x + 1)
2
3
− (4
2
4 − 8)
dx
dx
dx
1
32 − (2 x + 1)
2
dx
3
⎛ 2 x + 1⎞
arcsin
+c
⎝ 3 ⎠
2
Exercise 19D
1. Find the following:
1
(a) (i) ∫
dx
1 2x 2
1
(b) (i) ∫
dx
1 3x 2
(ii)
∫1
(ii)
∫
© Cambridge University Press 2012
1
dx
5x 2
1
dx
1 4x2
19 Further integration methods
643
(c) (i)
∫x
(d) (i)
∫
9
dx
+9
2
dx
25 − x 2
2
(ii)
∫x
(ii)
∫
10
dx
+ 10
5
dx
4 − x2
2
2. By first completing the square, find the following:
1
1
(a) (i) ∫ 2
dx
(ii) ∫ 2
dx
x + 4x 5
x 6 x 10
1
1
(b) (i) ∫
dx
(ii) ∫
dx
2
8
15
2x x 2
6
5
(c) (i) ∫ 2
dx
(ii) ∫
dx
2
x + 10x
10 x + 27
−44 x 12 x
3. Find the exact value of
∫
0
3 /2
3
dx .
1 4x2
[4 marks]
4 x + 11 in the form 2 ( x + p ) + q.
4x
3
(b) Hence find ∫ 2
dx .
2 x 4x
4 x + 11
2
4. (a) Write 2 x 2
5. (a) Write 1 6x
6 − 3x 2 in the form a2
(b) Hence find the exact value of ∫
2
1
6. Show that ∫
55
4x
3
2
[5 marks]
3( x − b) .
2
1
dx .
1 6 x − 3x 2
10
π
dx = .
24 x 61
4
[5 marks]
[6 marks]
7. By using the substitution u = e x , find the exact value of
∫
1
ln 3
2
0
1
e
x
e
x
dx .
[6 marks]
19E Other strategies for integrating
quotients
We now have a number of techniques to use when integrating
a quotient of functions; we can reverse a standard derivative,
apply the reverse chain rule or use an inverse trigonometric
function. However, there are occasions when none of these
methods seem to be useful. In such cases our only remaining
option is to split the fraction into two separate fractions, each of
which we can solve.
The easiest way to do this is to split the numerator.
644 Topic 6: Calculus
© Cambridge University Press 2012
Worked example 19.14
Find∫
x +1
dx .
1 − x2
Split the fraction into two
The first part can be integrated
using the reverse chain rule
or the substitution u 1 x 2
∫
x+1
x
x=∫
1− x
1− x
2
dx + ∫
1
1 − x2
dx
First integral:
du
= −2 x ⇒ d =
dx
x 1
= ∫−
du
u 2x
Let u = 1 − x 2, so
∴∫
x
1 − x2
= ∫−
1
du
2x
1 − 21
u du
2
= − u + c = − 1 − x2 + c
We recognise the second part as
the derivative of arcsin
∫
∴∫
1
1 − x2
x+1
dx = a c i x + c
1 − x2
dx = − 1 − x 2 + arcsin x + c
Sometimes it is not obvious how to split the numerator.
Worked example 19.15
Find ∫
4 x 19
dx .
x + 12x
12 x + 41
2
No obvious options, so split
the fraction into two
However, simply writing this as:
4x
19
+ 2
2
x + 12x
12 + 41 x + 12x
12 + 41
is no use as we still can’t integrate
Instead, make the numerator of the first
fraction a multiple of the derivative of the
denominator in order to apply the reverse
chain rule and then hope to be able to
deal with the resulting second fraction
© Cambridge University Press 2012
4 x 19
dx
+ 12x
12 x + 41
4 x 24
5
=∫ 2
x−∫ 2
dx
x 12x
12 x + 41
x + 12x
12 x + 41
∫x
2
19 Further integration methods
645
continued . . .
=∫
This works; second fraction can be
integrated with the arctan function
(after completing the square)
Apply reverse chain rule to the first
u′
fraction (it is of the form ) and
u
arctan to the second
4 + 24
x + 12 x + 41
= 2∫
2 + 12
x + 12 x + 41
= 2l
(
+
+
−∫
5
( x + 6)2 + 5
−∫
)−
dx
1
2
⎛ x + 6⎞
⎜⎝
⎟ +1
5 ⎠
dx
⎛ x + 6⎞
5 arctan ⎜
+c
⎝ 5 ⎟⎠
We should always check whether the fraction can be simplified
before trying to split the numerator.
Worked example 19.16
Integrate ∫
x+4
dx .
12 − 5x 2 x 2
Check whether the polynomial
factorises
We now have a standard integral,
just remember to divide by the
coefficient of x
You will need the
method of comparing coefficients from
Section 3A.
646 Topic 6: Calculus
x+4
∫ 12 − 5x
=
2x
x+4
dx
)(x + 4 )
(3 − 2x )(
1
=∫
dx
3 − 2x
x=∫
1
ln 3 − 2 x + c
2
The final type of functions to consider are improper fractions.
These can be integrated by splitting them into a polynomial plus
a proper fraction.
© Cambridge University Press 2012
Worked example 19.17
(a) Find constants A, B and C such that:
(b) Hence find
x2 + 5
∫ x+2
x2 + 5
c
= Ax + B +
x+2
x 2
dx .
(a) x 2 + 5
We can multiply both sides by x + 2 to get rid
of fractions
(
x
)( x
2) + C
When x = − 2:
Setting x = −2 will eliminate the first term on the
right, so we can find C
( 2 ) + 5 = ( −2A +
∴C = 9
x2 5
x2
x terms: 1 = A
To find A and B we need to expand
the brackets and compare coefficients
)(0)
2Ax + Bxx + 2
9
2
x terms: 0
2A + B = 2
2
So
x2 + 5
9
= x−2+
x+2
x+2
x2 + 5
9
(b) ∫
x ∫x−2+
dx
x+2
x+2
1
= x 2 − 2 + 9 ln x + 2 + C
2
The result from part (a) allows us to use
standard integrals
Exercise 19E
1. By first simplifying, find:
(a)
(4 2 9)2
∫ (2 3)2
dx
(b)
x+3
∫ 6 − 13 x
5x 2
dx
2. Find the following by splitting the numerator:
5 1
x −3
(a) ∫ 2
dx
dx
(b) ∫
x +6
4 − x2
© Cambridge University Press 2012
19 Further integration methods
647
(c)
∫x
2
8 23
dx
+ 8 x 25
(d)
∫
x −5
dx
− x + 6x − 7
2
3. Find the following by splitting into a polynomial and a
proper fraction:
x +1
2 3
dx
(ii) ∫
dx
(a) (i) ∫
x+2
x −1
x2 + 2
dx
x −3
x 2 + 5x
5x 1
dx
(c) ∫
2
x +3
(b) (i)
∫
4. (a) Show that
1
x 2
(ii)
=
∫
x 2 + 2x − 1
dx
x+5
1
5
= 2
.
x +3 x + x −6
5
dx giving your answer in the
x + x −6
form ln ( f ( x )) + c.
[5 marks]
(b) Hence find ∫
2
5. Find the exact value of ∫
2
0
6. (a) Show that
1
0
value of k.
5−x
dx =
2 + x − x2
k , find the
[6 marks]
4 5
dx .
1 − x2
8. (a) Write 2 x 2
[5 marks]
88xx + 17 in the form a ( x − p ) + q.
(b) Hence find ∫
648 Topic 6: Calculus
[4 marks]
5−x
1
2
=
+
.
2
2+ x −x
2 − x 1+ x
(b) Given that ∫
7. Find ∫
4
dx .
x +4
2
2
2
2x
2
8
dx .
88xx + 17
[7 marks]
© Cambridge University Press 2012
19F Integration by parts
In Section B we saw cases where we could integrate products of
functions by using the reverse chain rule or a substitution, but
we cannot yet solve integrals such as ∫ x sin x dx or ∫ x 2 e x dx .
In order to deal with these we return to the product rule for
differentiation (Key point 18.4).
d
dv
du
(uv ) = u + v
dx
dx
dx
Integrating with respect to x we get:
dv
du
uv = ∫ u
x + ∫v
dx
dx
dx
dv
du
⇒∫
= − ∫v
dx
dx
dx
KEY POINT 19.8
The integration by parts formula.
dv
du
∫ u dx dx uv ∫ v dx dx
When using integration by parts, the challenge is deciding
dv
which of the functions is to be u and which . The aim is to
dx
du
select them so that the product v
is easier to integrate than
dx
the original product. This often (but not always) means that you
choose u to be a polynomial.
Worked example 19.18
Find ∫ x sin x dx .
This is a product to which we can’t
apply the reverse chain rule,
so try integration by parts
Choose u to be the polynomial part
Apply the formula
u = x and
⇒
du
= 1 and v = − cos x
dx
dv
∫ u dx d
∫x
dv
= sin x
dx
i
u
uv
d
du
∫ v dx dx
x( co x)
∫(
co x) dx
= − x cos x + ∫ cos x dx
= − x cos x + sin x + c
© Cambridge University Press 2012
19 Further integration methods
649
It may be necessary to use integration by parts more than once.
As long as the integrals are becoming simpler each time, you are
on the right track! The next example also shows you how to deal
with the limits.
Worked example 19.19
Find the exact value of
∫
ln 2
0
x2
x
This is a product to which we
can’t apply the reverse chain
rule, so try integration by parts
Choose the polynomial as u
Apply the formula
Put in the limits on the uv part
straight away
dx .
u = x 2 and
⇒
du
= 2 x and v = e x
dx
dv
du
= uv ∫ v
∫ u dx
dx
dx
∫
ln 2
0
We have to integrate a product
again, so use integration by
parts again
Choose u to be the polynomial
again
dv
= ex
dx
x 2 e x dx
u = 2x and
⇒
[ x2 ex ]ln0 2 ∫0
ln 2
2 x e x dx
dv
= ex
dx
du
= 2 and v = e x
dx
dv
= 2x
dx
we would end up back where
we started!)
(If we used u = e x and
Apply the formula again and
use the limits
Put both integrals together,
making sure to keep track
of negative signs by using
brackets appropriately
650 Topic 6: Calculus
So,
∫
l 2
ln
0
2 xe x dx
[2xe ]ln0 2 ∫0
ln 2
Therefore,
∫
ln 2
0
x 2 e x dx
[ x2 ex ]ln0 2
=(
2e x dx = [
2
{[2xxe ]
x ln 2
0
−
)−(
]ln0 2 − [
ln 2
2 x ]0 }
[2e
− )+(
]ln0 2
−
)
= 2( 2) 2 − 4 l 2 + 2
© Cambridge University Press 2012
Sometimes it seems that we are getting nowhere, as the new
integral resulting from integration by parts is no easier than the
original. However, as long as things are not getting worse, they
will eventually get better as the following example shows.
Worked example 19.20
Use integration by parts to find ∫ e x cos x dx.
This is one of the rare occasions
when it makes no difference which
dv
way round we choose u and
dx
Applying the formula, the new
integral is no better but also no
worse than the original
dv
= ex
dx
u = cos x and
⇒
du
= − sin x and v = e x
dx
dv
∫ u dx
dx
∫e
x
du
∫ v dx
uv
cos x e x
cos x dx
= cos e x
With no other option, we proceed
with a second integration by parts
u = sin x and
We need to be consistent and
dv
in the same way
choose u and
dx
as before to avoid undoing what
we have just done
⇒
It seems that no progress has been
made as we have ended up with
the integral we started with (except
with a different sign)
However, when we put everything
together it becomes apparent
that the different sign allows us to
rearrange and find an expression
for ∫e x s x dx
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x
dx
x
dv
= ex
dx
du
= cos x and v = e x
dx
So,
∫ ex sin x dx
sin x e x
Therefore,
∫e
∫ ( i )e
+ ∫ e sin
i x dx
x
co
⇒ 2∫
⇒∫
d
x
x
co
ex
∫ co
e x dx
{ in x e
x
co
e x dx
}
d = e x cos x + e x i x
d =
ex
(cos x + i x ) + c
2
19 Further integration methods
651
We are able to differentiate and integrate e x but so far we have
only been able to differentiate ln x.
In order to be able to integrate ln x, we can use integration by
parts. This might not seem an obvious method at first because
there is no product of functions here, but with a little creativity
we can proceed.
Worked example 19.21
Find ∫ ln x dx.
The seemingly trivial step of writing
ln x as the product of 1 and ln x
sets up integration by parts
∫ ln x dx ∫
ln x dx
Cannot integrate lnx so let u lnx
u = ln x and
dv
=1
dx
⇒
Apply the parts formula
du 1
=
and v = x
dx x
dv
du
= uv ∫ v
∫ u dx
dx
dx
∫ 1 × ln
=(
) −∫
1
x dx
x
= x ln
l x − ∫ 1 dx
= x ln
l x−x+c
Although this example only shows how to integrate ln x, in most
other cases of integration by parts involving ln x we would still
let ln x = u. The choices for u and v in the common cases are
summarised below.
KEY POINT 19.9
When integrating ∫ x f (x ) dx by parts, choose u = xn in all
cases except when f(x) = ln x.
652 Topic 6: Calculus
© Cambridge University Press 2012
Exercise 19F
1. Use integration by parts to find the following:
⎛ x⎞
(a) (i)
∫ x cos2x dx
(ii)
∫ x sin ⎜⎝ 2 ⎟⎠ dx
(b) (i)
∫ 4 x dx
∫ 2 x l 5 x dx
∫ x cos 3x dx
(ii)
∫ x dx
∫ x ln x dx
∫ x sin x dx
−2x
2
(c) (i)
(ii)
2
(d) (i)
(ii)
4x
2
1 2 x4
∫ 4 x e dx
ln x
(f) ∫ 3 dx
x
(e)
(g)
∫ (ln x )
2
dx
2. Use integration by parts to find the following:
(a)
∫ arctan x dx
(b)
3. Evaluate the following exactly:
(a)
∫
π2
(b)
∫
2
(c)
∫
π3
0
1
0
∫ ln (2x 1) dx
x cos x dx
ln x
dx
x2
sin x ln (sec x ) dx
4. When using the integration by parts formula, we start
dv
with
and find v. Why do we not include a constant of
dx
integration when we do this? Try a few examples adding
+ C to v and see what happens.
5. Find ∫ x
−3x
3
dx .
[5 marks]
e
6. Evaluate ∫ x 5 ln x dx.
[6 marks]
1
7. (a) Show that ∫ tan x dx = ln sec x + c .
(b) Hence find
x
∫ cos
2
x
dx .
8. Find the value of k such that
[8 marks]
∫
k
0
arccos d
© Cambridge University Press 2012
0 .5
[7 marks]
19 Further integration methods
653
9. Use the substitution x + 1 u to find the
3 1
exact value of ∫ e 1 dx.
−1 2
[8 marks]
Summary
•
Look for standard derivatives before attempting any more complicated methods. They are
given in the Formula booklet, but you may need to divide by the coefficient of x.
•
If the expression contains both a function and its derivative and the ‘inside’ function is of the
form (ax + b) it is highly likely to be susceptible to the reverse chain rule or a substitution.
•
Integration by substitution can also work in other situations, and you need to be able to use
any given substitution. The steps of integration by substitution are given in Key point 19.2.
•
When evaluating a definite integral using substitution, see Key point 19.3.
•
A particular case of substitution, where the top of the fraction is the derivative of the bottom, is
worth remembering:
f ′( )
∫ f ( ) dxx = f ( ) + c
Many integrals involving trigonometric functions can be simplified using identities.
1
1
(1 cos2
cos 2x ) ; tan2 x sec
(1 cos2
Particularly useful identities are: cos2
s 2 x − 1.
cos 2x ) ; sin2
2
2
For example, to integrate sin2x, use cos2x = 1 – 2sin2x; to integrate cos2x, use cos2x = 2cos2x – 1;
d
to integrate tannx, use 1 + tan2x = sec2x and that
( tan x ) s 2 x.
dx
•
•
The integral of tan is worth knowing: ∫ tan x x = ln
l sec x + c.
•
Higher powers of sine and cosine can be integrated using De Moivre’s Theorem.
1
1
⎛ x⎞
Integration can be done using inverse trigonometric functions: ∫ 2
dx = arctan
+ c;
2
a x
a
⎝ a⎠
1
⎛ x⎞
∫ a2 x 2 dx = arcsin ⎝ a ⎠ + c x < a .
It may be necessary to split a fraction that contains two (or more) terms in the numerator into
two separate functions before integrating each with the above methods. If the degree of the
numerator is at least as large as the degree of the denominator, then write it as a polynomial
plus a proper fraction and compare coefficients.
•
(
•
)
•
It may be possible to integrate a product of functions using the integration by parts formula:
•
∫ u dx dx
•
The challenge with integration questions is often not in carrying out any of the above methods,
but actually in selecting the correct method to use in the first place. In the exam you will often,
but not always, be told which method to use. For extra practice see the Extension worksheet 17
‘Basic integration’ and the Extension worksheet 19 ‘Further integration methods’.
dv
uv
du
∫ v dx dx
654 Topic 6: Calculus
© Cambridge University Press 2012
Introductory problem revisited
Use integration to prove that the area of a circle of radius r is equal to πr2.
y
x2 + y 2 = r 2
r
(x, y)
r
y
r
x
−r
x
There is a small problem: the equation of the circle above is a
relation, not a function – the graph of a circle does not pass the
vertical line test.
−r
We can only integrate functions, but we can avoid the problem
by considering only the top half of the circle and then doubling
the answer we get for the area.
You might find
Key point 5.1 from
chapter 5 useful here.
y
(x, y)
r
−r
For the top half of the circle y > 0, so y
r 2 − x 2 . Now that
you have done lots of integration practice, you may suspect
that this one needs a substitution and it turns out that a useful
substitution is x = r cos θ. This makes some sense, as we know that
trigonometric functions are closely related to circles.
√
y = r 2 − x2
r
In order to use integration, we need to think of a circle as a
graph of a function. We saw at the beginning of chapter 18
that the coordinates of a point on the circle with radius r
satisfy the equation x2 + y2 = r2. (You are not required to know
this equation for the exam!) In chapter 18 we used implicit
differentiation to find the gradient; in order to integrate we need
an explicit expression for y in terms of x.
x
y
r
x
exam hint
This is not one of the standard integrals, so if you have to
do it in the exam you should be given a hint.
√
y = − r 2 − x2
−r
Now that we have decided on the strategy we can carry out the
integration.
Write down the integral to be
evaluated
State the method to be used
© Cambridge University Press 2012
The area of the top half of the circle is given by:
r
A
= ∫ r 2 − x 2 dx
−r
2
Substitution: x = r cos θ
19 Further integration methods
655
continued . . .
Differentiate
dx
= −r sinθ
dθ
⇒ d = − sinθ dθ
Express the integrand in terms of θ
r2
Notice that sin θ is positive on the
top half of the circle
⇒
Change the limits
Put everything together
Remove the minus sign by
swapping limits and take the
constant outside the integral
Use the double angle formula to
integrate sin2 θ
Hence the area of the whole circle is 2 ×
656 Topic 6: Calculus
(r
)2 = r 2 (
2
− x 2 = r sinθ
2
)
r 2 sin2 θ
Limits:
when x = −r , cos θ = −1 so θ = π
when x = r , cos θ = 1 so θ = 0
0
A
= ∫ (r
π
2
)( r
) dθ
π
= r 2 ∫ sin2 θ dθ
0
= r2 ∫
π
0
1 − cos 2θ
dθ
2
π
θ
2θ ⎤
= r 2 ⎡⎢ −
4 ⎥⎦ 0
⎣2
i 2π ⎤
⎧⎡ π
⎫
= r 2 ⎨⎢ −
− [0]⎬
⎥
4 ⎦
⎩⎣ 2
⎭
2
πr
=
2
πr 2
= πr 2, as required.
2
© Cambridge University Press 2012
Mixed examination practice 19
Short questions
π
1. Find the exact value of ∫ cos2 (3x ) dx .
[6 marks]
0
2. Use integration by parts to find ∫ x cos2 x dx.
m dx
3. Given that ∫
dx = 1 calculate, to 3 significant figures,
0 3x 1
the value of m.
π 12
1
4. Find the exact value of ∫
dx .
0
cos2 4 x
5. Find the following integrals:
1
1
(a) ∫
dx
dx
(b) ∫
1 3x
(2 3)2
[6 marks]
[6 marks]
[5 marks]
[6 marks]
6. Find ∫ ln x dx .
[5 marks]
7. (a) Simplify
[6 marks]
8. Find ∫
e −4 + e −2 x
e −4 + e −2 x
(b)
Hence
fi
nd
∫ e−4 x − 9 dx .
e −4 x − 9
6 4
dx .
x2 + 4
[5 marks]
x+5
2
1
can be written as
−
.
(x + 1)(x + 2)
x 1 x+2
(b) Hence find, in the form ln k, the exact value
7
x+5
of ∫
dx .
5 (x
)(
)( x + )
1
10. Find ∫
dx .
x ln x
1
11. Using the substitution u
x − 1, or otherwise,
2
x
find ∫
dx .
1
2 x −1
9. (a) Show that
12. Find the exact value of
∫
5
2
x −1
dx .
x+2
∫
a
−a
[5 marks]
[7 marks]
2
dx = 1, find the exact value of a.
1 − x2
© Cambridge University Press 2012
[6 marks]
[6 marks]
13. Use integration by parts to find ∫ arctan x dx .
14. Given that
[8 marks]
[7 marks]
19 Further integration methods
657
Long questions
4 3x
A
1 − Bx
can be written in the form
+ 2
2
(x + 2)(x 1)
x + 2 x +1
finding the constants A and B.
1. (a) Show that
(b) Hence find
∫(
4 3x
dx .
+ 2) ( 2 1)
(c) Find the exact value of ∫0
3 2
4 3x
dx .
1 − x2
sin x
cos x
2. Let I = ∫
dx and J = ∫
dx .
sin x + cos x
sin x + cos x
(a) Find I J .
[15 marks]
(b) By using the substitution u in x + cos x , find J
sin x
(c) Hence find ∫
dx .
sin x + cos
c x
I.
[9 marks]
⎛ x⎞
3. Let t = tan ⎜ ⎟ dx.
⎝ 2⎠
(a) Find
dt
in terms of t.
dx
(b) (i) Show that sin 2θ =
2 tanθ
.
sec2 θ
2t
.
1+ t2
x
(c) Use the substitution t = tan ⎛ ⎞ to evaluate
⎝ 2⎠
(ii) Hence show that sin x =
∫
π2
0
1
dx .
1 + sin x
[14 marks]
4. Consider the complex number z = cosθ + sinθ.
(a) Using De Moivre’s Theorem show that z n +
4
1⎞
⎛
(b) By expanding ⎜ z + ⎟ , show that cos 4 θ
⎝
z⎠
1
= 2cos nθ .
zn
1
(cos 4θ 4 cos 2θ
8
3).
a
(c) Let g (a ) = ∫ cos 4 θ dθ .
0
(i) Find g (a).
(ii) Solve g (a) = 1.
[11 marks]
(© IB Organization 2004)
658 Topic 6: Calculus
© Cambridge University Press 2012