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Calculus Level 5

0 1 ln ( x ) 1 x 2 d x \large \int_{0}^{1} \dfrac{\ln( x)}{1-x^2} \, dx

The above integral is equal to a π b c \dfrac{-a\pi^b}{c} for positive integers a , b , c a,b,c , for positive integers a , b a,b and c c with a a and c c coprime. Evaluate ( a + b + c ) 2 (a+b+c)^2 .

Note

You are given that ζ ( 2 ) = π 2 6 \zeta(2) = \dfrac{\pi^2}{6} .

Try my set .


The answer is 121.

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2 solutions

Kazem Sepehrinia
Jun 29, 2015

Let t = ln x t=\ln x I = 0 t e t 1 e 2 t d t = 0 t e t 1 e 2 t d t = 0 t e t ( n = 0 e 2 n t ) d t = 0 t ( n = 0 e ( 2 n + 1 ) t ) d t = n = 0 ( 0 t e ( 2 n + 1 ) t d t ) = n = 0 1 ( 2 n + 1 ) 2 = π 2 8 \begin{array}{c}\text{I} &= \int_{-\infty}^{0} \frac{t e^t}{1-e^{2t}} \text{d} t \\ &=\int_{0}^{\infty} \frac{t e^{-t}}{1-e^{-2t}} \text{d} t \\ &=\int_{0}^{\infty} t e^{-t} \left(\sum_{n=0}^{\infty} e^{-2nt} \right) \text{d} t \\ &=\int_{0}^{\infty} t \left(\sum_{n=0}^{\infty} e^{-(2n+1)t} \right) \text{d} t \\ &= \sum_{n=0}^{\infty} \left(\int_{0}^{\infty} te^{-(2n+1)t} \text{d} t \right) \\ &=-\sum_{n=0}^{\infty} \frac{1}{(2n+1)^2}\\ &=-\frac{\pi^2}{8} \end{array}

I don't undetstand when u replace 1/(1-e^-2t) by sum. Can u help me figure it out?

Khang Nguyễn - 5 years, 10 months ago

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For x < 1 |x|<1 1 1 x = 1 + x + x 2 + . . . \frac{1}{1-x}=1+x+x^2+...

Kazem Sepehrinia - 5 years, 10 months ago

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ah geometric series. Thank you !

Khang Nguyễn - 5 years, 10 months ago
Hassan Abdulla
May 1, 2018

0 < x < 1 1 1 x 2 = k = 0 x 2 k // geometric series I = 0 1 ln ( x ) 1 x 2 d x = 0 1 k = 0 x 2 k ln ( x ) d x = k = 0 0 1 x 2 k ln ( x ) d x I = k = 0 [ x 2 k + 1 2 k + 1 ln ( x ) 0 1 ] 1 2 k + 1 0 1 x 2 k d x = k = 0 1 ( 2 k + 1 ) 2 // integration by part I is only the odd squares so I = [ k = 1 1 k 2 k = 1 1 ( 2 k ) 2 ] = [ k = 1 1 k 2 1 4 k = 1 1 k 2 ] I = [ π 2 6 1 4 π 2 6 ] = π 2 8 0<x<1\Rightarrow \frac { 1 }{ 1-x^{ 2 } } = \sum _{ k=0 }^{ \infty }{ { x }^{ 2k } } \color{#3D99F6} \text{ // geometric series} \\ I=\int _{ 0 }^{ 1 } \frac { \ln { ( } x) }{ 1-x^{ 2 } } dx=\int _{ 0 }^{ 1 } \sum _{ k=0 }^{ \infty }{ { x }^{ 2k }\cdot \ln { ( } x) } dx=\sum _{ k=0 }^{ \infty }{ { \int _{ 0 }^{ 1 } x }^{ 2k }\cdot \ln { ( } x) } dx \\ I=\sum _{ k=0 }^{ \infty }{ { \left[ \frac { { x }^{ 2k+1 } }{ 2k+1 } \ln { ( } x)|_{ 0 }^{ 1 } \right] -\frac { 1 }{ 2k+1 } \int _{ 0 }^{ 1 }{ { x }^{ 2k }dx } } } =-\sum _{ k=0 }^{ \infty }{ \frac { 1 }{ { \left( 2k+1 \right) }^{ 2 } } } \color{#3D99F6} \text{ // integration by part} \\ \color{#D61F06} I \text{ is only the odd squares} \\ \text{so }I=-\left[ \sum _{ k=1 }^{ \infty }{ \frac { 1 }{ { k }^{ 2 } } } -\sum _{ k=1 }^{ \infty }{ \frac { 1 }{ { \left( 2k \right) }^{ 2 } } } \right] =-\left[ \sum _{ k=1 }^{ \infty }{ \frac { 1 }{ { k }^{ 2 } } } -\frac { 1 }{ 4 } \sum _{ k=1 }^{ \infty }{ \frac { 1 }{ { k }^{ 2 } } } \right] \\ I=-\left[ \frac { { \pi }^{ 2 } }{ 6 } -\frac { 1 }{ 4 } \cdot \frac { { \pi }^{ 2 } }{ 6 } \right] =-\frac { { \pi }^{ 2 } }{ 8 }

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