Let's do some calculus! (22)

Calculus Level 4

π / 2 π / 2 e sin x cos x 1 + e tan x d x = ? \large \displaystyle \int_{{-\pi}/{2}}^{{\pi}/{2}} {\dfrac{ e^{\left| \sin x \right|} \cos x}{1 + e^{\tan x}}} \,dx ~ = ~ ?

Notations: e 2.718 e \approx 2.718 is the Euler's number . For your answer take this approximate value as the value of e e .


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The answer is 1.718.

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1 solution

Chew-Seong Cheong
Sep 24, 2016

I = π / 2 π / 2 e sin x cos x 1 + e tan x d x = π / 2 0 e sin x cos x 1 + e tan x d x + 0 π / 2 e sin x cos x 1 + e tan x d x Let u = x , d x = d u = 0 π / 2 e sin ( u ) cos ( u ) 1 + e tan ( u ) d u + 0 π / 2 e sin x cos x 1 + e tan x d x = 0 π / 2 e sin u cos u 1 + e tan u d u + 0 π / 2 e sin x cos x 1 + e tan x d x Putting u = x = 0 π / 2 e sin x cos x ( 1 1 + e tan x + 1 1 + e tan x ) d x = 0 π / 2 e sin x cos x ( e tan x e tan x + 1 + 1 1 + e tan x 1 ) d x = 0 π / 2 e sin x cos x d x = e sin x 0 π / 2 = e 1 1.718 \begin{aligned} I & = \int_{-\pi/2}^{\pi/2} \frac {e^{|\sin x|}\cos x}{1+e^{\tan x}} dx \\ & = \int_{-\pi/2}^0 \frac {e^{\color{#D61F06}{-}\sin \color{#3D99F6}{x}}\cos \color{#3D99F6}{x}}{1+e^{\tan \color{#3D99F6}{x}}} d \color{#3D99F6}{x} + \int_0^{\pi/2} \frac {e^{\sin x}\cos x}{1+e^{\tan x}} dx & \small \color{#3D99F6}{\text{Let }u = -x, \ dx = - du} \\ & = \int^{\color{#3D99F6}{\pi/2}}_{\color{#3D99F6}{0}} \frac {e^{-\sin \color{#3D99F6}{(-u)}}\cos \color{#3D99F6}{(-u)}}{1+e^{\tan \color{#3D99F6}{(-u)}}} d \color{#3D99F6}{u} + \int_0^{\pi/2} \frac {e^{\sin x}\cos x}{1+e^{\tan x}} dx \\ & = \int^{\color{#3D99F6}{\pi/2}}_{\color{#3D99F6}{0}} \frac {e^{\sin \color{#3D99F6}{u}}\cos \color{#3D99F6}{u}}{1+e^{-\tan \color{#3D99F6}{u}}} d \color{#3D99F6}{u} + \int_0^{\pi/2} \frac {e^{\sin x}\cos x}{1+e^{\tan x}} dx & \small \color{#3D99F6}{\text{Putting }u=x } \\ & = \int_0^{\pi/2} e^{\sin x}\cos x \left(\frac 1{1+e^{-\tan x}} + \frac 1{1+e^{\tan x}} \right) dx \\ & = \int_0^{\pi/2} e^{\sin x}\cos x \left(\cancel{\frac {e^{\tan x}}{e^{\tan x}+1} + \frac 1{1+e^{\tan x}}}^1 \right) dx \\ & = \int_0^{\pi/2} e^{\sin x}\cos x \ dx \\ & = e^{\sin x}\bigg|_0^{\pi/2} = e - 1 \approx \boxed{1.718} \end{aligned}

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