4.7 Integration by Substitution

www.ck12.org. Chapter 4. Integration, Solution Key. 4.7 Integration by Substitution. 1. Let u = ln x and dv = x. Then du = 1 x dx and v = x2. 2 . ∫ x ln x dx = uv−lim ...
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Chapter 4. Integration, Solution Key

4.7 Integration by Substitution 1. Let u = ln x and dv = x. Then du = 1x dx and v =

Z

x2 2.

x ln x dx = uv − lim v du x2 ln x x2 1 − · dx 2 2 x Z x x2 ln x − dx = 2 2 x2 ln x x2 = − +C 2 4 Z

=

2. Let u = ln x and dv =

√ 3 x. Then du = 1x dx and v = 23 x 2 .

Z Z3

x ln x dx = uv −

Z

v du 3

3

Z √ 3 1 2x 2 ln x 2 xln x dx = − x 2 · dx 3 3 x

1

1 3 2

2x ln x 2 = − 3 3

Z3

1

x 2 dx 1

=

3 2

3

2x ln x 4x 2 − 3 9

! 3 1

3 2

3 2

3

3

  4 2 × 3 ln x 4 × 3 − − 0− = 3 9 9 2 × 3 2 ln x 4 × 3 2 4 = − + 3 9 9 3. Let u2 = 2x + 1. Then

u2 = 2x + 1 u2 − 1 = 2x u2 − 1 =x 2 u du = dx 157

4.7. Integration by Substitution

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Z

  1 u2 − 1 u du u 2 Z 2 u −1 = du 2 Z  1 u2 − 1 du = 2   1 u2 = −u 2 3 " √ # 3 √ 2x + 1 1 = − 2x + 1 2 3   1√ 2x + 1 = 2x + 1 − 1 +C 2 3 √ 2x + 1 (2x − 2) 1 = · +C 2√ 3 2x + 1 (x − 1) = +C 3

x √ dx = 2x + 1

Z

4. Let u = 1 − x2 . Then

du = −2x dx 1 − du = x dx 2 and u − 1 = −x2 − u + 1 = x2

When using u−substitution, just put limits as u1 and u2 as placeholders on the integral. After u is replaced by the function of x, put back the original limits of integration. 158

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Chapter 4. Integration, Solution Key

Z1

Zu2  p  p x 1 − x2 dx = x2 x 1 − x2 dx 3

u1

0

1 = 2 =

1 2

1 = 2

Zu2

− (−u + 1)

√ u du

u1 Zu2 

 1 3 u 2 − u 2 du

u1 5

u2 5 2

3



u2

!

3 2

  1 3 5   2 1 2 1 − x2 2 − 1 − x2 2 = 2 5 3 0   1 2 2 − = 0− 2 5 3 2 = 15 5. Let u = x and dv = cos x dx. Then du = dx and v = sin x. Z

x cos x dx = x sin x −

Z

sin x dx

= x sin x − (− cos x) +C = x sin x + cos x +C 6. Let u = x3 + 9. Then

du = 3x2 dx 1 du = x2 dx 3

Z1 0

Zu2 p 1√ 3 x x + 9dx = u du 3 2

u1

1 = 3

Zu2

1

u 2 du u1 3

1 u2 = 3 3 2 1 q 2 = (x3 + 9)3 9 0   2 √ 3 2 = 10 − 27 9 159

4.7. Integration by Substitution

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7. Let

1 = x−1 x du = −x−2 dx 1 −du = 2 dx x u=

Z 

1 1 ×ex 2 x



Z

eu du

dx =

= −eu +C 1

= e x +C 2

2

8. Let u = x2 and dv = xex dx. Then du = 2x dx and v = 12 ex . Z

9. Let u = ln x and dv =

1 3 x2

3

2 2 1 x3 ex dx = x2 ex (2x) dx 2 Z 2 2 1 = x2 ex − ex (x) dx 2 2 1 1 2 = x2 ex − ex +C 2 2

dx = x− 2 dx. Then du =

Z

1 x

−1

and v = − 23 x 2 .

 −3  Z  −2  x −32 −2 ln x x 2 − dx 5 dx = 3 3 x x2 Z 5 −2ln x 2 = + x− 2 dx −3 3 3x 2

ln x

3

2 x− 2  +C = + −3 3 − 32 3x 2 −2ln x 4 = − −3 +C −3 3x 2 9x 2 −2ln x

10.

Re 1 e x dx = ln x|1 = ln e − ln 0 = ln e = 1 1

160