# 6. Integration 6.2 Substitution Why

```6. Integration
6.2 Substitution
Why
Example 1. Getting acquainted: Find the following indefinite integral
Z
x5 (x6 + e7 )8 dx
How
Based on the reverse chain rule.
Recall of chain rule:
Let u = g(x) = x6 + e7 . Then, the differential of u
du =
So,
dx =
Thus,
Z
x5 (x6 + e7 )8 dx
1
Check (by differentiation as we did in 6.1):
What
The Substitution Rule: Let u = g(x), we have
Z
Z
0
f (g(x))g (x) dx = f (u) du,
assuming that u = g(x) is a continuously differentiable function whose range is an interval on which f
is continuous.
Trick
Choosing the right u in the integrand. It depends on the integral. Practice for gaining experience.
SOME PRACTICE. Finding the following indefinite integrals.
Example 2.
Z
x(−x2 + 9)2014 dx
Example 3.
Z
√
x 2 + 5x2 dx
2
Example 4.
Z
x − ln2
dx
(x2 − 2xln2)2
Example 5.
x − ln2
dx
− 2xln2
Z
x2
Example 6.
Z
e2x
dx
ex + e
3
Example 7.
Z
(1 − x)2 ex
3 −3x2 +3x−1
Example 8.
Z
2x
√
dx
3
x−π
4
dx
Example 9.
Z
provided
Z
√
1
x2 + 1
1
√
dx,
1 + 9x
√
dx = ln x + x2 + 1 + C.
Example 10.
Z r
x
dx
2−x
5
Example 11.
Z √
1 − x2 dx
More examples
6
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