Who's up to the challenge? 19

Calculus Level 5

0 1 ( ln x ) 3 ln ( 1 x 2 ) d x \large \int_0^1 ( \ln x)^3 \ln (1-x^2) \, dx

The value the integral above is equal to

A B ζ ( C ) D F π G + H π J K L ln 2 , -\dfrac { A }{ B } \zeta (C)-\dfrac { D }{ F } { \pi }^{ G }+H-\dfrac { \pi ^{ J } }{ K } -L\ln { 2 },

where A , B , C , D , F , G , H , J , K A,B,C,D,F,G,H,J,K and L L are positive integers with gcd ( A , B ) = gcd ( D , F ) = 1 \gcd(A,B) = \gcd(D,F) = 1 .

Find A + B + C + D + F + G + H + J + K + L A+B+C+D+F+G+H+J+K+L .


this is a part of Who's up to the challenge?


The answer is 105.

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1 solution

Neelesh Vij
Jul 10, 2017

Using expansion for l n ( 1 x 2 ) ln(1-x^2)

I = 0 1 ( l n x ) 3 ( k = 1 x 2 k k ) d x \displaystyle -I = \int_{0}^{1} (lnx)^3 \left( \sum_{k=1}^{ \infty } \dfrac{x^{2k}}{k} \right) dx

Interchanging sings of summation and integration

I = k = 1 1 k 0 1 ( l n x ) 3 x 2 k d x \displaystyle -I= \sum_{k=1}^{ \infty } \dfrac{1}{k} \int_{0}^{1} (lnx)^3 x^{2k} dx

Integrating by parts 3 times we get

I 6 = k = 1 1 k ( 2 k + 1 ) 4 \dfrac{-I}{6} = \sum_{k=1}^{ \infty } \dfrac{1}{k(2k+1)^4}

Using partial fractions we get

I 6 = k = 1 ( 2 ( 2 k + 1 ) 4 + 2 ( 2 k + 1 ) 3 + 2 ( 2 k + 1 ) 2 + 2 2 k ( 2 k + 1 ) ) \displaystyle \dfrac{-I}{6} = \sum_{k=1}^{ \infty } \left( \dfrac{-2}{(2k+1)^4} +\dfrac{-2}{(2k+1)^3} + \dfrac{-2}{(2k+1)^2} +\dfrac{2}{2k(2k+1)} \right)

Now sums are easy. solving them we get

I = 21 2 ζ ( 3 ) 3 π 2 2 + 48 π 4 8 12 ln ( 2 ) \displaystyle I = \dfrac{-21}{2} \zeta{(3)} - \dfrac{3 \pi^2}{2} + 48 - \dfrac{ \pi^4}{8} -12 \ln(2)

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