Even or Odd

Calculus Level 3

π 4 π 4 sin ( 2 x ) ln ( 1 + e x ) ( 2 + cos ( 2 x ) ) 2 d x = ? \large \int_{-\frac{\pi}{4}}^{\frac{\pi}{4}}\frac{\sin(2x)\ln(1+e^x)}{(2+\cos(2x))^2}\mathrm dx = \ ?


The answer is 0.04519959382.

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1 solution

Mark Hennings
Apr 27, 2020

Firstly I = 1 4 π 1 4 π sin 2 x ln ( 1 + e x ) ( 2 + cos 2 x ) 2 d x = 1 4 π 1 4 π sin 2 x ( 2 + cos 2 x ) 2 [ 1 2 x + ln ( e 1 2 x + e 1 2 x ) ] d x = 1 2 1 4 π 1 4 π x sin 2 x ( 2 + cos 2 x ) 2 d x I \; = \; \int_{-\frac14\pi}^{\frac14\pi} \frac{\sin2x \ln(1 + e^x)}{(2 + \cos2x)^2}\,dx \; = \; \int_{-\frac14\pi}^{\frac14\pi} \frac{\sin2x}{(2 + \cos2x)^2}\big[\tfrac12x + \ln(e^{\frac12x} + e^{-\frac12x})\big]\,dx \; = \; \frac12\int_{-\frac14\pi}^{\frac14\pi} \frac{x\sin2x}{(2 + \cos2x)^2}\,dx since sin 2 x ln ( e 1 2 x + e 1 2 x ) ( 2 + cos 2 x ) 2 \frac{\sin2x \ln(e^{\frac12x}+e^{-\frac12x})}{(2 + \cos2x)^2} is an odd function. Now integrating by parts gives I = 1 4 [ x 2 + cos 2 x ] 1 4 π 1 4 π 1 4 1 4 π 1 4 π d x 2 + cos 2 x = π 16 1 4 1 4 π 1 4 π d x 2 + cos 2 x = π 16 1 8 1 2 π 1 2 π d x 2 + cos x I \; = \; \frac14\left[ \frac{x}{2 + \cos2x}\right]_{-\frac14\pi}^{\frac14\pi} - \frac14\int_{-\frac14\pi}^{\frac14\pi} \frac{dx}{2 + \cos2x} \; = \; \frac{\pi}{16} - \frac14\int_{-\frac14\pi}^{\frac14\pi} \frac{dx}{2 + \cos2x} \; = \; \frac{\pi}{16} - \frac18\int_{-\frac12\pi}^{\frac12\pi} \frac{dx}{2 + \cos x} Finally, the substitution t = tan 1 2 x t = \tan\tfrac12x gives I = π 16 1 4 1 1 d t 3 + t 2 = π 16 1 4 3 [ tan 1 t 3 ] 1 1 = π 16 π 12 3 = π 144 ( 9 4 3 ) = 0.045199593829843926 I \; = \; \frac{\pi}{16} - \frac{1}{4}\int_{-1}^1 \frac{dt}{3 + t^2} \; = \; \frac{\pi}{16} - \frac{1}{ 4\sqrt{3}}\left[\tan^{-1}\tfrac{t}{\sqrt{3}}\right]_{-1}^1 \; = \; \frac{\pi}{16} - \frac{\pi}{12\sqrt{3}} \; = \; \boxed{\frac{\pi}{144}(9 - 4\sqrt{3}) \; = \; 0.045199593829843926}

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