Somewhat Interesting

Calculus Level 3

1 2014 2014 arctan ( x ) x d x = ? \large \int_{\frac1{2014}}^{2014} \frac{ \text{arctan}(x)}{x} \ dx = \ ?

1 4 π ln ( 2014 ) \frac 14 \pi \ln(2014) 1 2 π ln ( 2014 ) \frac 12 \pi \ln(2014) π ln ( 2014 ) \pi \ln(2014)

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1 solution

Otto Bretscher
May 4, 2015

Beautiful and interesting problem!

We make a substitution x = 1 y x=\frac{1}{y} , with d x = 1 y 2 d y dx=-\frac{1}{y^2}dy , and find 1 / 2014 2014 arctan x x d x = 1 / 2014 2014 arctan ( 1 / y ) y d y \int_{1/2014}^{2014}\frac{\arctan{x}}{x}dx = \int_{1/2014}^{2014}\frac{\arctan(1/y)}{y}dy = 1 / 2014 2014 ( π 2 arctan y ) y d y = π ln 2014 1 / 2014 2014 arctan x x d x = \int_{1/2014}^{2014}\frac{(\frac{\pi}{2}-\arctan{y})}{y}dy=\pi\ln{2014}-\int_{1/2014}^{2014}\frac{\arctan{x}}{x}dx

We solve and find 1 / 2014 2014 arctan x x d x = π 2 ln ( 2014 ) \int_{1/2014}^{2014}\frac{\arctan{x}}{x}dx=\boxed{\frac{\pi}{2}\ln(2014)}

Thumbs up @Otto Bretscher sir

Karan Arora - 6 years, 1 month ago

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