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Questions  >>  CBSE XII  >>  Math  >>  Integrals
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Q)

Evaluate:\[\int \limits _0 ^ \pi \Large\frac{e^{\cos x}}{[e^{\cos x}+e^{-\cos x}]}dx\]

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A)
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  • (i) $\int \limits_a^b f(x)dx=F(b)-F(a)$
  • (ii) $ \int \limits_a^b f(x)dx=\int \limits_a^b f(a-x) dx$
  • (iii) $ \cos (\pi-x)=\cos x$
Given $\int \limits _0 ^ \pi \Large\frac{e^{\cos x}}{e^{\cos x}+e^{-\cos x}}dx$------(1)
 
By applying the property $ \int \limits_a^b f(x)dx=\int \limits_a^b f(a-x) dx$
 
$I=\int \limits _0 ^ \pi \Large\frac{e^{\cos (\pi-x)}}{e^{\cos (\pi-x)}+e^{-\cos (\pi-x)}}dx$
 
But $\cos (\pi-x)=-\cos x $
 
Therefore $I=\int \limits _0 ^ \pi \Large\frac{e^{-\cos x}}{e^{-\cos x}+e^{-(-\cos x)}}dx=\int \limits _0 ^ \pi \Large\frac{e^{-\cos x}}{e^{-\cos x}+e^{\cos x}}dx$------(2)
 
Adding equ(1) and equ (2)
 
$2I=\int \limits _0 ^ \pi \Large\frac{e^{\cos x}}{e^{\cos x}+e^{-\cos x}}+\frac{e^{-\cos x}}{e^{-\cos x}+e^{\cos x}}dx$
 
$=\int \limits _0 ^ \pi \Large\frac{e^{\cos x}+e ^{-\cos x}}{e^{\cos x}+e^{-\cos x}}dx$
 
$=\int \limits _0 ^ \pi dx$
 
on integrating we get
 
$2I=[x]_0^\pi$
 
on applying limits
 
$2I=\pi-0=\pi$
 
Therefore $ I=\frac{\pi}{2}$

 

 

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