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Definite Integration question

2012 · Shift 2 · Q27
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  5. /2012 · Shift 2 · Q27

Definite Integration question

2012 · Shift 2 · Q27

JEE AdvancedMathematicsDefinite IntegrationMCQ+4 / −1
The value of the integral ∫−π/2π/2(x2+1nπ+xπ−x)cos⁡xdx\int\limits_{ - \pi /2}^{\pi /2} {\left( {{x^2} + 1n{{\pi + x} \over {\pi - x}}} \right)\cos xdx}−π/2∫π/2​(x2+1nπ−xπ+x​)cosxdx is
  1. A
    000
  2. B
    π22−4{{{\pi ^2}} \over 2} - 42π2​−4
  3. C
    π22+4{{{\pi ^2}} \over 2} + 42π2​+4
  4. D
    π22{{{\pi ^2}} \over 2}2π2​
View written solutionFree

Correct answer: B

  1. Interpret the integral

The given integral is

I=∫−π/2π/2(x2+ln⁡π+xπ−x)cos⁡x dx.I=\int_{-\pi/2}^{\pi/2}\left(x^2+\ln\frac{\pi+x}{\pi-x}\right)\cos x\,dx.I=∫−π/2π/2​(x2+lnπ−xπ+x​)cosxdx.

We split it as

I=I1+I2,I=I_1+I_2,I=I1​+I2​,

where

I1=∫−π/2π/2x2cos⁡x dx,I2=∫−π/2π/2ln⁡π+xπ−xcos⁡x dx.I_1=\int_{-\pi/2}^{\pi/2} x^2\cos x\,dx, \qquad I_2=\int_{-\pi/2}^{\pi/2} \ln\frac{\pi+x}{\pi-x}\cos x\,dx.I1​=∫−π/2π/2​x2cosxdx,I2​=∫−π/2π/2​lnπ−xπ+x​cosxdx.
  1. Evaluate I1I_1I1​

Notice that x2x^2x2 is even and cos⁡x\cos xcosx is even, so x2cos⁡xx^2\cos xx2cosx is even. Hence

I1=2∫0π/2x2cos⁡x dx.I_1=2\int_0^{\pi/2} x^2\cos x\,dx.I1​=2∫0π/2​x2cosxdx.

Now integrate by parts:

Let

u=x2,dw=cos⁡x dx.u=x^2,\quad dw=\cos x\,dx.u=x2,dw=cosxdx.

Then

du=2x dx,w=sin⁡x.du=2x\,dx,\quad w=\sin x.du=2xdx,w=sinx.

So

∫x2cos⁡x dx=x2sin⁡x−∫2xsin⁡x dx.\int x^2\cos x\,dx=x^2\sin x-\int 2x\sin x\,dx.∫x2cosxdx=x2sinx−∫2xsinxdx.

Again integrate by parts on ∫2xsin⁡x dx\int 2x\sin x\,dx∫2xsinxdx:

∫xsin⁡x dx=−xcos⁡x+sin⁡x.\int x\sin x\,dx=-x\cos x+\sin x.∫xsinxdx=−xcosx+sinx.

Thus

∫x2cos⁡x dx=x2sin⁡x+2xcos⁡x−2sin⁡x.\int x^2\cos x\,dx=x^2\sin x+2x\cos x-2\sin x.∫x2cosxdx=x2sinx+2xcosx−2sinx.

Now evaluate from 000 to π/2\pi/2π/2:

∫0π/2x2cos⁡x dx=[x2sin⁡x+2xcos⁡x−2sin⁡x]0π/2.\int_0^{\pi/2} x^2\cos x\,dx =\left[x^2\sin x+2x\cos x-2\sin x\right]_0^{\pi/2}.∫0π/2​x2cosxdx=[x2sinx+2xcosx−2sinx]0π/2​.

At x=π/2x=\pi/2x=π/2,

sin⁡π2=1,cos⁡π2=0,\sin\frac\pi2=1,\quad \cos\frac\pi2=0,sin2π​=1,cos2π​=0,

so value is

π24−2.\frac{\pi^2}{4}-2.4π2​−2.

At x=0x=0x=0, value is 000.

Therefore

∫0π/2x2cos⁡x dx=π24−2,\int_0^{\pi/2} x^2\cos x\,dx=\frac{\pi^2}{4}-2,∫0π/2​x2cosxdx=4π2​−2,

and hence

I1=2(π24−2)=π22−4.I_1=2\left(\frac{\pi^2}{4}-2\right)=\frac{\pi^2}{2}-4.I1​=2(4π2​−2)=2π2​−4.
  1. Evaluate I2I_2I2​ using symmetry

Consider

f(x)=ln⁡π+xπ−x.f(x)=\ln\frac{\pi+x}{\pi-x}.f(x)=lnπ−xπ+x​.

Then

f(−x)=ln⁡π−xπ+x=−ln⁡π+xπ−x=−f(x).f(-x)=\ln\frac{\pi-x}{\pi+x}=-\ln\frac{\pi+x}{\pi-x}=-f(x).f(−x)=lnπ+xπ−x​=−lnπ−xπ+x​=−f(x).

So f(x)f(x)f(x) is odd.

Also, cos⁡x\cos xcosx is even. Therefore,

f(x)cos⁡xf(x)\cos xf(x)cosx

is odd.

The integral of an odd function over [−a,a][-a,a][−a,a] is 000. Hence

I2=∫−π/2π/2ln⁡π+xπ−xcos⁡x dx=0.I_2=\int_{-\pi/2}^{\pi/2} \ln\frac{\pi+x}{\pi-x}\cos x\,dx=0.I2​=∫−π/2π/2​lnπ−xπ+x​cosxdx=0.
  1. Combine the results

Thus

I=I1+I2=(π22−4)+0=π22−4.I=I_1+I_2=\left(\frac{\pi^2}{2}-4\right)+0=\frac{\pi^2}{2}-4.I=I1​+I2​=(2π2​−4)+0=2π2​−4.
  1. Check options

The value is

π22−4.\boxed{\frac{\pi^2}{2}-4}.2π2​−4​.

This matches Option B.


  1. Compare with stored correct answer

Stored correct answer: B

Our derived answer: B

So they agree.

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