Integral Integral11

Calculus Level pending

64 ( 1 ( 0 log ( 1 x k + 1 ) d x ) 2 sin ( 2 π k ) π k ) d k = a ( b + b + b + b b ) π c \int_{64}^{\infty } \left(\frac{1}{\left(\int_0^{\infty } \log \left(\frac{1}{x^k}+1\right) \, dx\right){}^2}-\frac{\sin \left(\frac{2 \pi }{k}\right)}{\pi k}\right) \, dk=\frac{a \left(\sqrt{b+\sqrt{b+\sqrt{b+\sqrt{b}}}}-b\right)}{\pi ^c}

where a , b , c a,b,c are positive integers. Submit a + b + c a+b+c .


The answer is 20.

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1 solution

Hint:

If y 1 y \geq 1 ,

y ( 1 ( 0 log ( 1 x k + 1 ) d x ) 2 sin ( 2 π k ) π k ) d k = y sin 2 ( π y ) π 2 \int_y^{\infty } \left(\frac{1}{\left(\int_0^{\infty } \log \left(\frac{1}{x^k}+1\right) \, dx\right){}^2}-\frac{\sin \left(\frac{2 \pi }{k}\right)}{\pi k}\right) \, dk=-\frac{y \sin ^2\left(\frac{\pi }{y}\right)}{\pi ^2}

using that

0 log ( 1 x k + 1 ) d x = π csc ( π k ) \int_0^{\infty } \log \left(\frac{1}{x^k}+1\right) \, dx=\pi \csc \left(\frac{\pi }{k}\right)

for k > 1 k>1 .

Note that sin 2 ( π y ) \sin ^2\left(\frac{\pi }{y}\right) is easy to write in radicals at y = 64 y=64 , since 64 is a power of 2.

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