Integral?

Calculus Level 4

0 1 l n ( x ) 1 x 2 d x \int_{0}^{1} \frac{ln(x)}{1-x^{2}} dx

-pi^2/4 pi^2/4 pi -pi^2/8

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2 solutions

[ Int ( a ) = 0 t x a 1 x 2 d x = 0 t ( n = 0 x a + 2 n ) d x = n = 0 0 t x a + 2 n d x = n = 0 t a + 2 n + 1 a + 2 n + 1 ] \text{}\left[\text{Int}(a)=\int_0^t \frac{x^a}{1-x^2} \, dx=\int_0^t \left(\sum _{n=0}^{\infty } x^{a+2 n}\right) \, dx=\sum _{n=0}^{\infty } \int_0^t x^{a+2 n} \, dx=\sum _{n=0}^{\infty } \frac{t^{a+2 n+1}}{a+2 n+1}\right]

[ Int ( a ) = 0 t x a log ( x ) 1 x 2 d x = n = 0 ( log ( t ) t a + 2 n + 1 a + 2 n + 1 t a + 2 n + 1 ( a + 2 n + 1 ) 2 ) ] \text{}\left[\text{Int}'(a)=\int_0^t \frac{x^a \log (x)}{1-x^2} \, dx=\sum _{n=0}^{\infty } \left(\frac{\log (t) t^{a+2 n+1}}{a+2 n+1}-\frac{t^{a+2 n+1}}{(a+2 n+1)^2}\right)\right]

Int ( 0 ) = 0 t log ( x ) 1 x 2 d x = ( n = 0 t 2 n + 1 ( 2 n log ( t ) + log ( t ) 1 ) ( 2 n + 1 ) 2 ) \text{Int}'(0)=\int_0^t \frac{\log (x)}{1-x^2} \, dx= \left(\sum _{n=0}^{\infty } \frac{t^{2 n+1} (2 n \log (t)+\log (t)-1)}{(2 n+1)^2}\right)

0 1 log ( x ) 1 x 2 d x = n = 0 1 ( 2 n + 1 ) 2 = π 2 8 \int_0^1 \frac{\log (x)}{1-x^2} \, dx =\sum _{n=0}^{\infty } -\frac{1}{(2 n+1)^2}=-\frac{\pi ^2}{8}

Adib Batous
Mar 8, 2016

How do you know that the final sum is equal to π 2 8 \frac{-\pi^{2}}{8} ?

Daniel Ellesar - 4 years, 4 months ago

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