More fun in 2015, Part 19

Calculus Level 3

( 1 π 0 sin ( x 2015 ) x d x ) 1 = ? \large \left(\frac{1}{\pi}\int_{0}^{\infty}\frac{\sin(x^{2015})}{{x}}dx\right)^{-1} = \ ?

P.S : It helps to use the Dirichlet Integral 0 sin x x d x = π 2 . \displaystyle\int_{0}^{\infty}\frac{\sin{x}}{x}dx=\frac{\pi}{2}.


The answer is 4030.

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1 solution

Sean Sullivan
Sep 20, 2015

Let u = x 2015 u=x^{2015} , then d u = 2015 x 2014 d x d u 2015 x 2015 = d x x d u 2015 u = d x x du=2015x^{2014}dx\implies\frac{du}{2015x^{2015}}=\frac{dx}{x}\implies\frac{du}{2015u}=\frac{dx}{x} :

Substituting into ( 1 π 0 sin ( x 2015 ) x d x ) 1 = ( 1 π 0 sin ( x 2015 ) d x x ) 1 \left(\frac{1}{\pi}\int_{0}^{\infty}\frac{\sin(x^{2015})}{{x}}dx\right)^{-1}=\left(\frac{1}{\pi}\int_{0}^{\infty}\sin(x^{2015})\frac{dx}{x}\right)^{-1}

we get

( 1 π 0 sin ( u ) 2015 u d u ) 1 = ( 1 π 1 2015 π 2 ) 1 = 4030 \left(\frac{1}{\pi}\int_{0}^{\infty}\frac{\sin(u)}{2015u}du\right)^{-1}=\left(\frac{1}{\pi}\cdot\frac{1}{2015}\cdot\frac{\pi}{2}\right)^{-1}=\boxed{4030}

Yes, exactly! Thanks!

Otto Bretscher - 5 years, 8 months ago

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