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Differential Equations question

2020 · 4 Sep · Shift 1 · Q38
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  5. /2020 · 4 Sep · Shift 1 · Q38

Differential Equations question

2020 · 4 Sep · Shift 1 · Q38

JEE MainMathematicsDifferential EquationsMCQ+4 / −1
Let y = y(x) be the solution of the differential equation, xy'- y = x2(xcosx + sinx), x > 0. if y (π\piπ) = π\piπ then y′′(π2)+y(π2)y''\left( {{\pi \over 2}} \right) + y\left( {{\pi \over 2}} \right)y′′(2π​)+y(2π​) is equal to :
  1. A
    1+π21 + {\pi \over 2}1+2π​
  2. B
    2+π2+π242 + {\pi \over 2} + {{{\pi ^2}} \over 4}2+2π​+4π2​
  3. C
    2+π22 + {\pi \over 2}2+2π​
  4. D
    1+π2+π241 + {\pi \over 2} + {{{\pi ^2}} \over 4}1+2π​+4π2​
View written solutionFree

Correct answer: C

  1. Given differential equation

xy′−y=x2(xcos⁡x+sin⁡x),x>0xy' - y = x^2(x\cos x + \sin x), \qquad x>0xy′−y=x2(xcosx+sinx),x>0

We rewrite it in linear form by dividing by xxx:

y′−1xy=x(xcos⁡x+sin⁡x)=x2cos⁡x+xsin⁡xy' - \frac{1}{x}y = x(x\cos x + \sin x)=x^2\cos x + x\sin xy′−x1​y=x(xcosx+sinx)=x2cosx+xsinx

  1. Identify integrating factor

For y′+P(x)y=Q(x),y' + P(x)y = Q(x),y′+P(x)y=Q(x), we have P(x)=−1xP(x)=-\frac{1}{x}P(x)=−x1​ so the integrating factor is

I.F.=e∫−1xdx=e−ln⁡x=1x\text{I.F.}=e^{\int -\frac{1}{x}dx}=e^{-\ln x}=\frac{1}{x}I.F.=e∫−x1​dx=e−lnx=x1​ (since x>0x>0x>0).

  1. Solve the differential equation

Multiply throughout by 1x\frac{1}{x}x1​:

1xy′−1x2y=xcos⁡x+sin⁡x\frac{1}{x}y' - \frac{1}{x^2}y = x\cos x + \sin xx1​y′−x21​y=xcosx+sinx

The left side is:

ddx(yx)=xcos⁡x+sin⁡x\frac{d}{dx}\left(\frac{y}{x}\right)=x\cos x + \sin xdxd​(xy​)=xcosx+sinx

Integrate:

yx=∫(xcos⁡x+sin⁡x) dx+C\frac{y}{x}=\int (x\cos x + \sin x)\,dx + Cxy​=∫(xcosx+sinx)dx+C

Now,

∫xcos⁡x dx=xsin⁡x+cos⁡x\int x\cos x\,dx = x\sin x + \cos x∫xcosxdx=xsinx+cosx by integration by parts, and

∫sin⁡x dx=−cos⁡x\int \sin x\,dx = -\cos x∫sinxdx=−cosx

Therefore,

∫(xcos⁡x+sin⁡x)dx=xsin⁡x+cos⁡x−cos⁡x=xsin⁡x\int (x\cos x + \sin x)dx = x\sin x + \cos x - \cos x = x\sin x∫(xcosx+sinx)dx=xsinx+cosx−cosx=xsinx

So,

yx=xsin⁡x+C\frac{y}{x}=x\sin x + Cxy​=xsinx+C

Hence,

y=x2sin⁡x+Cxy=x^2\sin x + Cxy=x2sinx+Cx

  1. Use the condition y(π)=πy(\pi)=\piy(π)=π

Substitute x=πx=\pix=π:

y(π)=π2sin⁡π+Cπ=0+Cπ=πy(\pi)=\pi^2\sin \pi + C\pi = 0 + C\pi = \piy(π)=π2sinπ+Cπ=0+Cπ=π

Thus,

C=1C=1C=1

So the solution is

y=x2sin⁡x+xy=x^2\sin x + xy=x2sinx+x

  1. Find y′′(π/2)y''(\pi/2)y′′(π/2) and y(π/2)y(\pi/2)y(π/2)

First derivative:

y′=ddx(x2sin⁡x+x)=2xsin⁡x+x2cos⁡x+1y' = \frac{d}{dx}(x^2\sin x + x)=2x\sin x + x^2\cos x + 1y′=dxd​(x2sinx+x)=2xsinx+x2cosx+1

Second derivative:

y′′=ddx(2xsin⁡x+x2cos⁡x+1)y'' = \frac{d}{dx}(2x\sin x + x^2\cos x + 1)y′′=dxd​(2xsinx+x2cosx+1)

Compute termwise:

ddx(2xsin⁡x)=2sin⁡x+2xcos⁡x\frac{d}{dx}(2x\sin x)=2\sin x + 2x\cos xdxd​(2xsinx)=2sinx+2xcosx

ddx(x2cos⁡x)=2xcos⁡x−x2sin⁡x\frac{d}{dx}(x^2\cos x)=2x\cos x - x^2\sin xdxd​(x2cosx)=2xcosx−x2sinx

Therefore,

y′′=2sin⁡x+4xcos⁡x−x2sin⁡xy'' = 2\sin x + 4x\cos x - x^2\sin xy′′=2sinx+4xcosx−x2sinx

At x=π2x=\frac{\pi}{2}x=2π​, we use sin⁡π2=1,cos⁡π2=0\sin \frac{\pi}{2}=1, \qquad \cos \frac{\pi}{2}=0sin2π​=1,cos2π​=0

So,

y′′(π2)=2+0−π24=2−π24y''\left(\frac{\pi}{2}\right)=2+0-\frac{\pi^2}{4}=2-\frac{\pi^2}{4}y′′(2π​)=2+0−4π2​=2−4π2​

Also,

y\left(\frac{\pi}{2}\right)=\left(\frac{\pi}{2}\right)^2\sin\frac{\pi}{2}+\frac{\pi}{2}= rac{\pi^2}{4}+\frac{\pi}{2}

  1. Add them

y′′(π2)+y(π2)=(2−π24)+(π24+π2)y''\left(\frac{\pi}{2}\right)+y\left(\frac{\pi}{2}\right)=\left(2-\frac{\pi^2}{4}\right)+\left(\frac{\pi^2}{4}+\frac{\pi}{2}\right)y′′(2π​)+y(2π​)=(2−4π2​)+(4π2​+2π​)

=2+π2=2+\frac{\pi}{2}=2+2π​

  1. Match with options

2+π22+\frac{\pi}{2}2+2π​ corresponds to Option C.

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