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Differential Equations question

2019 · 12 Jan · Shift 1 · Q45
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  5. /2019 · 12 Jan · Shift 1 · Q45

Differential Equations question

2019 · 12 Jan · Shift 1 · Q45

JEE MainMathematicsDifferential EquationsMCQ+4 / −1
Let y = y(x) be the solution of the differential equation, x dydx{{dy} \over {dx}}dxdy​+ y = x loge x, (x > 1). If 2y(2) = loge 4 −-− 1, then y(e) is equal to :
  1. A
    −e2- {e \over 2}−2e​
  2. B
    −e22- {{{e^2}} \over 2}−2e2​
  3. C
    e24{{{e^2}} \over 4}4e2​
  4. D
    e4{e \over 4}4e​
View written solutionFree

Correct answer: D

  1. Given differential equation

    xdydx+y=xln⁡x,x>1x\frac{dy}{dx}+y=x\ln x, \qquad x>1xdxdy​+y=xlnx,x>1

  2. Rewrite in standard linear form

    Divide throughout by xxx:

    dydx+1xy=ln⁡x\frac{dy}{dx}+\frac{1}{x}y=\ln xdxdy​+x1​y=lnx

    This is a linear differential equation:

    dydx+P(x)y=Q(x)\frac{dy}{dx}+P(x)y=Q(x)dxdy​+P(x)y=Q(x)

    where

    P(x)=1x,Q(x)=ln⁡xP(x)=\frac{1}{x}, \qquad Q(x)=\ln xP(x)=x1​,Q(x)=lnx

  3. Find the integrating factor

    I.F.=e∫1x dx=eln⁡x=x\text{I.F.}=e^{\int \frac{1}{x}\,dx}=e^{\ln x}=xI.F.=e∫x1​dx=elnx=x

    Since x>1x>1x>1, we can take I.F.=x\text{I.F.}=xI.F.=x.

  4. Multiply the equation by the integrating factor

    xdydx+y=xln⁡xx\frac{dy}{dx}+y=x\ln xxdxdy​+y=xlnx

    Observe that the left side is:

    ddx(xy)=xln⁡x\frac{d}{dx}(xy)=x\ln xdxd​(xy)=xlnx

  5. Integrate both sides

    xy=∫xln⁡x dx+Cxy=\int x\ln x\,dx + Cxy=∫xlnxdx+C

    Now evaluate:

    ∫xln⁡x dx\int x\ln x\,dx∫xlnxdx

    Use integration by parts:

    Let u=ln⁡x,dv=x dxu=\ln x, \quad dv=x\,dxu=lnx,dv=xdx Then du=1xdx,v=x22du=\frac{1}{x}dx, \quad v=\frac{x^2}{2}du=x1​dx,v=2x2​

    So,

    ∫xln⁡x dx=x22ln⁡x−∫x22⋅1x dx\int x\ln x\,dx=\frac{x^2}{2}\ln x-\int \frac{x^2}{2}\cdot \frac{1}{x}\,dx∫xlnxdx=2x2​lnx−∫2x2​⋅x1​dx

    =x22ln⁡x−12∫x dx=\frac{x^2}{2}\ln x-\frac{1}{2}\int x\,dx=2x2​lnx−21​∫xdx

    =x22ln⁡x−x24=\frac{x^2}{2}\ln x-\frac{x^2}{4}=2x2​lnx−4x2​

    Hence,

    xy=x22ln⁡x−x24+Cxy=\frac{x^2}{2}\ln x-\frac{x^2}{4}+Cxy=2x2​lnx−4x2​+C

    Therefore,

    y=x2ln⁡x−x4+Cxy=\frac{x}{2}\ln x-\frac{x}{4}+\frac{C}{x}y=2x​lnx−4x​+xC​

  6. Use the initial condition

    Given:

    2y(2)=ln⁡4−12y(2)=\ln 4-12y(2)=ln4−1

    So,

    y(2)=ln⁡4−12y(2)=\frac{\ln 4-1}{2}y(2)=2ln4−1​

    Substitute x=2x=2x=2 into the general solution:

    y(2)=22ln⁡2−24+C2y(2)=\frac{2}{2}\ln 2-\frac{2}{4}+\frac{C}{2}y(2)=22​ln2−42​+2C​

    y(2)=ln⁡2−12+C2y(2)=\ln 2-\frac{1}{2}+\frac{C}{2}y(2)=ln2−21​+2C​

    Also,

    ln⁡4−12=2ln⁡2−12=ln⁡2−12\frac{\ln 4-1}{2}=\frac{2\ln 2-1}{2}=\ln 2-\frac{1}{2}2ln4−1​=22ln2−1​=ln2−21​

    Therefore,

    ln⁡2−12+C2=ln⁡2−12\ln 2-\frac{1}{2}+\frac{C}{2}=\ln 2-\frac{1}{2}ln2−21​+2C​=ln2−21​

    which gives

    C2=0  ⟹  C=0\frac{C}{2}=0 \implies C=02C​=0⟹C=0

  7. Find y(e)y(e)y(e)

    Now,

    y=x2ln⁡x−x4y=\frac{x}{2}\ln x-\frac{x}{4}y=2x​lnx−4x​

    At x=ex=ex=e:

    y(e)=e2ln⁡e−e4y(e)=\frac{e}{2}\ln e-\frac{e}{4}y(e)=2e​lne−4e​

    Since ln⁡e=1\ln e=1lne=1,

    y(e)=e2−e4=e4y(e)=\frac{e}{2}-\frac{e}{4}=\frac{e}{4}y(e)=2e​−4e​=4e​

  8. Option check

    • A: −e2-\frac{e}{2}−2e​ ❌
    • B: −e22-\frac{e^2}{2}−2e2​ ❌
    • C: e24\frac{e^2}{4}4e2​ ❌
    • D: e4\frac{e}{4}4e​ ✅

Therefore, the correct answer is D.

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