Integral 1

Calculus Level 5

Evaluate:

0 π / 2 sin x cos x tan x ln ( tan x ) d x \large \int_{0}^{\pi/2} \sin x \cos x \sqrt{\tan x}\ln (\tan x) \, dx

The answer can be expressed as π ( b π ) c a \dfrac{\pi\left(b-\pi\right)\sqrt c}a , where a a , b b , and c c are positive integers and c c is square-free; find 100 a + 10 b + c 100a+10b+c .


The answer is 1642.

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2 solutions

Mark Hennings
May 17, 2017

The substitution t = tan 2 x t = \tan^2x gives I = 0 1 2 π sin x cos x tan x ln ( tan x ) d x = 1 4 0 t 1 4 ( t + 1 ) 2 ln t d t I \; = \; \int_0^{\frac12\pi} \sin x \cos x \sqrt{\tan x} \ln(\tan x)\,dx \; = \; \tfrac14\int_0^\infty \frac{t^{\frac14}}{(t+1)^2}\ln t\, dt and hence I = 1 4 ( B a ( a , b ) B b ( a , b ) ) a = 5 4 , b = 3 4 = 1 4 B ( a , b ) ( ψ ( a ) ψ ( b ) ) a = 5 4 , b = 3 4 = 1 4 B ( 5 4 , 3 4 ) [ ψ ( 5 4 ) ψ ( 3 4 ) ] = Γ ( 5 4 ) Γ ( 3 4 ) 4 Γ ( 2 ) [ 4 + ψ ( 1 4 ) ψ ( 3 4 ) ] = Γ ( 1 4 ) Γ ( 3 4 ) 16 [ 4 + ψ ( 1 4 ) ψ ( 1 4 ) π ] = π 2 16 ( π 4 ) = π ( 4 π ) 2 16 \begin{aligned} I & = \; \tfrac14\left(\frac{\partial B}{\partial a}(a,b) - \frac{\partial B}{\partial b}(a,b)\right)\Big|_{a=\frac54,b=\frac34} \\ & = \tfrac14B(a,b)(\psi(a) - \psi(b)) \Big|_{a=\frac54,b=\frac34} \; = \; \tfrac14B\big(\tfrac54,\tfrac34\big)\big[\psi\big(\tfrac54\big) - \psi\big(\tfrac34\big)\big] \\ & = \frac{\Gamma(\frac54)\Gamma(\frac34)}{4\Gamma(2)} \big[4 + \psi(\tfrac14) - \psi(\tfrac34)\big] \; = \; \frac{\Gamma(\frac14)\Gamma(\frac34)}{16}\big[4 + \psi(\tfrac14) - \psi(\tfrac14) - \pi\big] \\ & = \frac{\pi\sqrt{2}}{16}(\pi-4) \; =\; \frac{\pi(4-\pi)\sqrt{2}}{16} \end{aligned} making the answer 100 × 16 + 10 × 4 + 2 = 1642 100\times16 + 10\times4 + 2 = \boxed{1642} .

Anton Wu
May 18, 2017

Use the u u -sub, u = tan x d u = sec 2 x 2 tan x d x u=\sqrt{\tan{x}}\to\,du=\frac{\sec^2{x}}{2\sqrt{\tan{x}}}\,dx :

I = 0 π / 2 sin x cos x tan x ln ( tan x ) d x = 4 0 π / 2 sec 2 x tan 2 x 2 sec 4 x tan x ln ( tan x ) d x = 4 0 u 4 ln u ( u 4 + 1 ) 2 d u I=\int_{0}^{\pi/2} {\sin{x}\cos{x}\sqrt{\tan{x}}\ln{\left(\tan{x}\right)}\,dx}=4\int_{0}^{\pi/2} {\frac{\sec^2{x}\tan^2{x}}{2\sec^4{x}\sqrt{\tan{x}}}\ln{\left(\sqrt{\tan{x}}\right)}\,dx}=4\int_{0}^{\infty} {\frac{u^4 \ln{u}}{{\left(u^4 + 1\right)}^2}\,du}

This integral can be solved using complex integration, with a keyhole contour; but a simpler, strictly-real solution introduces the integral:

F ( a ) = 0 d x x a + 1 = 0 0 e t e t x a d t d x = 1 a Γ ( 1 a ) Γ ( 1 1 a ) = π a csc ( π a ) F(a)=\int_{0}^{\infty} {\frac{\,dx}{x^a+1}}=\int_{0}^{\infty} {\int_{0}^{\infty} {e^{-t}e^{-tx^a}\,dt\,dx}}=\frac{1}{a}\Gamma\left(\frac{1}{a}\right)\Gamma\left(1-\frac{1}{a}\right)=\frac{\pi}{a}\csc{\left(\frac{\pi}{a}\right)}

Then, by Feynman's trick of 'differentiation under the integral sign':

F ( a ) = d d a 0 d x x a + 1 = 0 x a ln x d x ( x a + 1 ) 2 = 1 a ( π a csc ( π a ) ) ( 1 π a cot ( π a ) ) F'(a)=\frac{\,d}{\,da}\int_{0}^{\infty} {\frac{\,dx}{x^a+1}}=\int_{0}^{\infty} {\frac{x^a \ln{x}\,dx}{\left(x^a+1\right)^2}}=\frac{1}{a}\left(\frac{\pi}{a}\csc{\left(\frac{\pi}{a}\right)}\right)\left(1-\frac{\pi}{a}\cot{\left(\frac{\pi}{a}\right)}\right)

Thus, the integral is:

I = 4 F ( 4 ) = 4 ( 1 4 ( π 4 csc ( π 4 ) ) ( 1 π 4 cot ( π 4 ) ) ) = 1 16 π ( 4 π ) 2 I=4F'(4)=4\left(\frac{1}{4}\left(\frac{\pi}{4}\csc{\left(\frac{\pi}{4}\right)}\right)\left(1-\frac{\pi}{4}\cot{\left(\frac{\pi}{4}\right)}\right)\right)=\boxed{\frac{1}{16}\pi\left(4-\pi\right)\sqrt{2}}

and the answer is 100 16 + 10 4 + 2 = 1642 100\cdot16+10\cdot4+2=1642 .

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