A calculus problem by Hamza A

Calculus Level 4

0 1 ln ( x 2 ) x 2 1 d x = π a b \large \displaystyle\int _{ 0 }^{ 1 }{ \dfrac { \ln {({ x }^ 2) } }{ { x }^{ 2 }-1 } dx } =\dfrac { { \pi }^{ a } }{ b }

If the equation above holds true for positive integers a a and b b , find a × b a\times b .


The answer is 8.

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3 solutions

First Last
Apr 19, 2016

Rewriting using partial fractions as 2 0 1 ln x 2 ( x 1 ) d x + ln x 2 ( x + 1 ) d x \displaystyle 2\int_{0}^{1}\frac{\ln{x}}{2(x-1)}dx+\frac{-\ln{x}}{2(x+1)}dx

0 1 ln x ( x + 1 ) d x = [ ln ( x ) ln ( x + 1 ) ] 0 1 0 1 ln ( x + 1 ) x d x = 0 π 2 12 \displaystyle\int_{0}^{1}\frac{\ln{x}}{(x+1)}dx = [ \ln(x) \ln(x+1)]^1_0 - \int_{0}^{1}\frac{\ln(x+1)}{x}dx = 0 - \frac{\pi^2}{12}

0 1 ln x ( x 1 ) d x = π 2 6 \displaystyle\int_{0}^{1}\frac{\ln{x}}{(x-1)}dx = \frac{\pi^2}{6} , both by expanding and integrating the power series for ln ( x ) \ln(x) about x = 1 x=1

π 2 6 + π 2 12 = π 2 4 \displaystyle\frac{\pi^2}{6}+\frac{\pi^2}{12} = \boxed{\frac{\pi^2}{4}}

Aareyan Manzoor
May 29, 2017

I = 0 1 ln ( x 2 ) 1 x 2 d x = 0 1 n = 0 x 2 n ln x 2 d x = n = 0 0 1 x 2 n ln x 2 d x = n = 0 d d n 0 1 x 2 n d x = n = 0 d d n 1 2 n + 1 = n = 0 2 ( 2 n + 1 ) 2 = n = 1 2 n 2 n = 1 2 ( 2 n ) 2 = 2 π 2 6 π 2 12 = π 2 4 I=-\int_0^1 \dfrac{\ln(x^2)}{1-x^2} dx= -\int_0^1 \sum_{n=0}^\infty x^{2n} \ln{x^2}dx=-\sum_{n=0}^\infty\int_0^1 x^{2n} \ln{x^2}dx\\ =-\sum_{n=0}^\infty\dfrac{d}{dn} \int_0^1 x^{2n}dx=\sum_{n=0}^\infty-\dfrac{d}{dn} \dfrac{1}{2n+1}=\sum_{n=0}^\infty \dfrac{2}{(2n+1)^2}\\=\sum_{n=1}^\infty \dfrac{2}{n^2}-\sum_{n=1}^\infty \dfrac{2}{(2n)^2}=\dfrac{2\pi^2}{6}-\dfrac{\pi^2}{12}=\dfrac{\pi^2}{4}

Mihir Mallick
Feb 14, 2018

Since the integral is from 0 0 to 1 1 , we must have x 1 x≤1 . After using partial fractions, the integral splits into two integrals,each of which could be written as an infinite geometric series and which when calculated, give ζ ( 2 ) \zeta(2) and η ( 2 ) \eta(2) respectively. So the answer is ζ ( 2 ) \zeta(2) + η ( 2 ) \eta(2) = ζ ( 2 ) ( 1 + 1 2 1 ) \zeta(2)(1+1-2^{-1}) = π 2 4 \frac{\pi^2}{4} . Hence answer is 2 4 2*4 = 8 \boxed{8}

(I'm not actually showing that the two integrals obtained after using partial fractions evaluate to ζ ( 2 ) \zeta(2) and η ( 2 ) \eta(2) because there are many steps involved (including LHopital's rule). But the answer was indeed having this surprisingly closed expression in terms of zeta and eta). :)

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