All Roads Lead To π \pi

Calculus Level 3

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

If the value of the integral above equals π a ! a + 1 \dfrac{\pi ^{a!}}{a+1} , where a a is a positive integer, find a a .


The answer is 2.

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1 solution

Arkin Dharawat
Feb 7, 2016

We can write the above Integral as an integral of an infinite series,like so:

0 1 ( ln x ) 2 ( 1 x ) 2 d x = 0 1 ( ln x ) 2 ( 1 + 2 x + 3 x 2 + 4 x 3 . . . . . . ) d x \int _{ 0 }^{ 1 }{ ({ \ln { x) } }^{ 2 }{ (1-x) }^{ -2 } } dx=\int _{ 0 }^{ 1 }{ { (\ln { x) } }^{ 2 }(1+2x+3{ x }^{ 2 }+4{ x }^{ 3 }......) } dx

A general form would be ,

I = r = 0 0 1 ( l n x ) 2 ( r + 1 ) x r d x I=\sum _{ r=0 }^{ \infty }{ \int _{ 0 }^{ 1 }{ { (lnx) }^{ 2 } } } (r+1){ x }^{ r }dx

now I ( m , n ) = 0 1 x m ( l n x ) n d x = ( 1 ) n n ! ( m + 1 ) n + 1 I(m,n)=\int _{ 0 }^{ 1 }{ { x }^{ m } } { (lnx) }^{ n }dx=\frac { { (-1 })^{ n }n! }{ { (m+1) }^{ n+1 } }

The derivation of the above expression is part of the solution of an amazing and very similar problem by Ishan Tarunesh,here's the link: Off the charts!

Therefore, On solving further

r = 0 ( r + 1 ) ( 1 ) 2 2 ! ( r + 1 ) 2 + 1 = 2 r = 0 1 ( r + 1 ) 2 = π 2 3 \sum _{ r=0 }^{ \infty }{ (r+1)\frac { { (-1) }^{ 2 }2! }{ { (r+1) }^{ 2+1 } } } \quad \quad =\quad \quad 2\sum _{ r=0 }^{ \infty }{ \frac { 1 }{ { (r+1 })^{ 2 } } \quad \quad = } \frac { { \pi }^{ 2 } }{ 3 }

And so a = 2 a=\boxed{2}

Fantastic

:--))

arjun gupta - 5 years, 4 months ago

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