Let's go back a couple years

Calculus Level 3

π / 2 π / 2 1 200 7 x + 1 sin 2008 x sin 2008 x + cos 2008 x d x = ? \large \int_{-\pi/2}^{\pi /2} \dfrac1{2007^x + 1} \cdot \dfrac{\sin^{2008} x}{\sin^{2008}x + \cos^{2008} x} \, dx = \, ?

π 4 \frac { \pi }{ 4 } π 2 \frac { \pi }{ 2 } None of the others ln 2 \ln { \sqrt { 2 } }

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2 solutions

Raj Rajput
Feb 1, 2016

When did you learn these? (I am a math enthusiast from class 10!)

Akhash Raja Raam - 5 years, 4 months ago

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I learnt integration in class 12 CBSE :)

RAJ RAJPUT - 5 years, 4 months ago

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Oh(Phew!!) Thanks!

Akhash Raja Raam - 5 years, 3 months ago

Where are you take such questions?

B.L. Dara - 5 years, 4 months ago

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I am not able to understand your question, please elaborate :)

RAJ RAJPUT - 5 years, 4 months ago
Chew-Seong Cheong
Mar 22, 2019

I = π 2 π 2 1 200 7 x + 1 sin 2008 x sin 2008 x + cos 2008 x d x = π 2 0 1 200 7 x + 1 sin 2008 x sin 2008 x + cos 2008 x d x + 0 π 2 1 200 7 x + 1 sin 2008 x sin 2008 x + cos 2008 x d x Replace x with x = 0 π 2 1 200 7 x + 1 sin 2008 x sin 2008 x + cos 2008 x d x + 0 π 2 1 200 7 x + 1 sin 2008 x sin 2008 x + cos 2008 x d x = 0 π 2 200 7 x 1 + 200 7 x sin 2008 x sin 2008 x + cos 2008 x d x + 0 π 2 1 200 7 x + 1 sin 2008 x sin 2008 x + cos 2008 x d x \begin{aligned} I & = \int_{-\frac \pi 2}^\frac \pi 2 \frac 1{2007^x+1} \cdot \frac {\sin^{2008}x}{\sin^{2008}x+\cos^{2008}x} dx \\ & = {\color{#3D99F6}\int_{-\frac \pi 2}^0 \frac 1{2007^x+1} \cdot \frac {\sin^{2008}x}{\sin^{2008}x+\cos^{2008}x} dx} + \int_0^\frac \pi 2 \frac 1{2007^x+1} \cdot \frac {\sin^{2008}x}{\sin^{2008}x+\cos^{2008}x} dx & \small \color{#3D99F6} \text{Replace }x \text{ with }-x \\ & = {\color{#3D99F6}\int_0^\frac \pi 2 \frac 1{2007^{-x}+1} \cdot \frac {\sin^{2008}x}{\sin^{2008}x+\cos^{2008}x} dx} + \int_0^\frac \pi 2 \frac 1{2007^x+1} \cdot \frac {\sin^{2008}x}{\sin^{2008}x+\cos^{2008}x} dx \\ & = \int_0^\frac \pi 2 \frac {2007^x}{1+2007^x} \cdot \frac {\sin^{2008}x}{\sin^{2008}x+\cos^{2008}x} dx + \int_0^\frac \pi 2 \frac 1{2007^x+1} \cdot \frac {\sin^{2008}x}{\sin^{2008}x+\cos^{2008}x} dx \end{aligned}

= 0 π 2 sin 2008 x sin 2008 x + cos 2008 x d x Using a b f ( x ) d x = a b f ( a + b x ) d x = 1 2 0 π 2 ( sin 2008 x sin 2008 x + cos 2008 x + cos 2008 x cos 2008 x + sin 2008 x ) d x = 1 2 0 π 2 d x = π 4 \begin{aligned} \ \ \ & = \int_0^\frac \pi 2 \frac {\sin^{2008}x}{\sin^{2008}x+\cos^{2008}x} dx & \small \color{#3D99F6} \text{Using }\int_a^b f(x)\ dx = \int_a^b f(a+b-x)\ dx \\ & = \frac 12 \int_0^\frac \pi 2 \left(\frac {\sin^{2008}x}{\sin^{2008}x+\cos^{2008}x} + \frac {\cos^{2008}x}{\cos^{2008}x+\sin^{2008}x} \right) dx \\ & = \frac 12 \int_0^\frac \pi 2 dx = \boxed {\dfrac \pi 4} \end{aligned}

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