C2: Chapter 5 Binomial
Expansion
Dr J Frost (jfrost@tiffin.kingston.sch.uk)
Last modified: 22nd September 2013
Starter
a)
b)
c)
d)
e)
Expand (a + b)0
Expand (a + b)1
Expand (a + b)2
Expand (a + b)3
Expand (a + b)4
1a3
1a4 +
1a
1a2 +
+ 3a2b
4a3b +
1?
?+ 1b
2ab
+ b2
?
?+ 3ab2 + 1b3
6a
? 2b2 + 4ab3 + b4
What do you notice about:
The coefficients:
The powers of a and b:
They follow Pascal’s triangle.?
Power of a decreases each time (starting at the power)
? (starting at 0)
Power of b increases each time
Quickfire Pascal
What coefficients in your expansion do you use if the power is:
2: 1 2 1
?
Quickfire Pascal
What coefficients in your expansion do you use if the power is:
4: 1 4 6 ? 4 1
Quickfire Pascal
What coefficients in your expansion do you use if the power is:
3: 1 3 3 ? 1
Quickfire Pascal
What coefficients in your expansion do you use if the power is:
5: 1 5 10 ? 10 5 1
Quickfire Pascal
What coefficients in your expansion do you use if the power is:
2: 1 2 1
?
Quickfire Pascal
What coefficients in your expansion do you use if the power is:
4: 1 4 6 ? 4 1
Quickfire Pascal
What coefficients in your expansion do you use if the power is:
3: 1 3 3 ? 1
Quickfire Pascal
What coefficients in your expansion do you use if the power is:
5: 1 5 10 ? 10 5 1
Quickfire Pascal
What coefficients in your expansion do you use if the power is:
4: 1 4 6 ? 4 1
Binomial Expansion
(x + 2y)4 =
1 x4
+ 4 x3(2y)
+ 6 x2(2y)2 + 4 x (2y)3 + 1 (2y)4
= x4 + 8x3y + 24x2y2 + 32xy3 + 16y4
Step 1: You could first put in the first term with decreasing powers.
Step 2: Put in your second term with increasing powers, starting from 0
(i.e. so that 2y doesn’t appear in the first term of the expansion, because the power is 0)
Step 3: Add the coefficients according to Pascal’s Triangle.
Your go...
(2x – 5)3
= (2x)3 + 3(2x)2(-5) + 3(2x)(-5)2 + (-5)3
?
3
2
= 8x – 60x + 150x – 125
Bro Tip: If one of the terms in the bracket is negative, the terms in
the result will oscillate between positive and negative.
The coefficient of x2 in the expansion of (2 – cx)3 is 294.
Find the possible value(s) of the constant c.
(2 – cx)3 = 23 + 3 22(-cx) + 3 21(-cx)2 + ...
So coefficient of x2 is 6c2?= 294
c=7
Bro Tip: When asked about a particular term, it’s helpful to write out the first few terms of the
expansion, until you write up to the one needed. There’s no point of simplifying the whole expansion!
Exercises
Page 79 Exercise 5A
1c, d, g, h
2d, g
3, 4, 6
How are the rows of Pascal’s Triangle generated?
How many ways are there of choosing 0 items from 4?
= 4C0 = 𝟒𝟎 ?= 𝟏
How many ways are there of choosing 1 item from 4?
= 4C1 = 𝟒𝟏 ?= 𝟒
How many ways are there of choosing 2 items from 4?
= 4C2 = 𝟒𝟐 ?= 𝟔
How many ways are there of choosing 3 items from 4?
= 4C3 = 𝟒𝟑 ?= 𝟒
How many ways are there of choosing 4 items from 4?
= 4C4 = 𝟒𝟒 ?= 𝟏
1
1
1
1
1
2
3
1
3
1
1 4 6 4 1
1 5 10 10 5 1
Binomial Coefficients
This is known as a binomial coefficient. It can also be written as nCr
(said: “n choose r”)
?
?
?
?
?
?
?
?
Binomial Coefficients
To calculate Binomial Coefficients easily:
Because when we divide 8! by 6!, we cancel out
all the numbers between 1 and 6 in the product.
i.e. The bottom number of the binomial
coefficient (2) tells us how many consecutive
numbers we multiply together.
?
?
?
?
?
General formula
Edexcel May 2013 (Retracted)
?
Using Binomial Expansions for approximations
Edexcel Jan 2012
?
If
, then x = 0.1. Plugging this in to our expansion:
1 + 0.2 + 0.0175 + 0.00875 = 1.218375
Actual value is (1.025)8 = 1.218403. So it is correct to 4dp!
Using Binomial Expansions for approximations
Exercise 5C
Q7
Write down the first four terms in the expansion of
By substituting an appropriate value for x, find an approximate value to (0.99)6.
Use your calculator to determine the degree of accuracy of your approximation.
1 – 0.6x + 0.15x2 – 0.02x3
0.94148, which is accurate to 5dp
Q8
?
Write down the first four terms in the expansion of
By substituting an appropriate value of x, find an approximate value to (2.1)10.
Use your calculator to determine your approximation’s degree of accuracy.
1024 + 1024x + 460.8x2 + 122.88x3
1666.56, which is accurate to 3sf
?
Using Binomial Expansions for approximations
Edexcel January 2007
a) 1 + 5(-2x) + 10(-2x)2 + 10(-2x)3
?
= 1 – 10x + 40x2 – 80x3
b) We discard the x2 and x3 terms above.
(1+x)(1-10x) = 1 – 10x + x – 10x
? 2 = 1 2– 9x – 10x2
 1- 9x (since we can discard the x term again)