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Differential Equations question

2023 · 29 Jan · Shift 2 · Q24
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  5. /2023 · 29 Jan · Shift 2 · Q24

Differential Equations question

2023 · 29 Jan · Shift 2 · Q24

JEE MainMathematicsDifferential EquationsMCQ+4 / −1
Let y=y(x)y=y(x)y=y(x) be the solution of the differential equation xlog⁡exdydx+y=x2log⁡ex,(x>1)x{\log _e}x{{dy} \over {dx}} + y = {x^2}{\log _e}x,(x \gt 1)xloge​xdxdy​+y=x2loge​x,(x>1). If y(2)=2y(2) = 2y(2)=2, then y(e)y(e)y(e) is equal to
  1. A
    1+e22{{1 + {e^2}} \over 2}21+e2​
  2. B
    1+e24{{1 + {e^2}} \over 4}41+e2​
  3. C
    2+e22{{2 + {e^2}} \over 2}22+e2​
  4. D
    4+e24{{4 + {e^2}} \over 4}44+e2​
View written solutionFree

Correct answer: D

  1. Given differential equation

    xln⁡x dydx+y=x2ln⁡x,x>1x\ln x\,\frac{dy}{dx}+y=x^2\ln x, \qquad x>1xlnxdxdy​+y=x2lnx,x>1

    with initial condition

    y(2)=2.y(2)=2.y(2)=2.

  2. Rewrite in linear form

    Divide both sides by xln⁡xx\ln xxlnx:

    dydx+1xln⁡xy=x.\frac{dy}{dx}+\frac{1}{x\ln x}y=x.dxdy​+xlnx1​y=x.

    This is a linear differential equation of the form

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

    where

    P(x)=1xln⁡x,Q(x)=x.P(x)=\frac{1}{x\ln x}, \qquad Q(x)=x.P(x)=xlnx1​,Q(x)=x.

  3. Find the integrating factor

    I.F.=e∫P(x)dx=e∫1xln⁡xdx.\text{I.F.}=e^{\int P(x)dx}=e^{\int \frac{1}{x\ln x}dx}.I.F.=e∫P(x)dx=e∫xlnx1​dx.

    Since

    ∫1xln⁡xdx=ln⁡(ln⁡x),\int \frac{1}{x\ln x}dx=\ln(\ln x),∫xlnx1​dx=ln(lnx),

    we get

    I.F.=eln⁡(ln⁡x)=ln⁡x\text{I.F.}=e^{\ln(\ln x)}=\ln xI.F.=eln(lnx)=lnx

    (valid because x>1⇒ln⁡x>0x>1 \Rightarrow \ln x>0x>1⇒lnx>0).

  4. Multiply the equation by the integrating factor

    ln⁡x dydx+yx=xln⁡x.\ln x\,\frac{dy}{dx}+\frac{y}{x}=x\ln x.lnxdxdy​+xy​=xlnx.

    The left-hand side is

    ddx(yln⁡x).\frac{d}{dx}(y\ln x).dxd​(ylnx).

    Hence,

    ddx(yln⁡x)=xln⁡x.\frac{d}{dx}(y\ln x)=x\ln x.dxd​(ylnx)=xlnx.

  5. Integrate both sides

    yln⁡x=∫xln⁡x dx+C.y\ln x=\int x\ln x\,dx + C.ylnx=∫xlnxdx+C.

    Use integration by parts for ∫xln⁡x dx\int x\ln x\,dx∫xlnxdx:

    Let u=ln⁡x,dv=x dx.u=\ln x, \quad dv=x\,dx.u=lnx,dv=xdx. Then du=1xdx,v=x22.du=\frac{1}{x}dx, \quad v=\frac{x^2}{2}.du=x1​dx,v=2x2​.

    So,

    ∫xln⁡x dx=x22ln⁡x−∫x22⋅1xdx\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+C.=\frac{x^2}{2}\ln x-\frac{x^2}{4}+C.=2x2​lnx−4x2​+C.

    Therefore,

    yln⁡x=x22ln⁡x−x24+C.y\ln x=\frac{x^2}{2}\ln x-\frac{x^2}{4}+C.ylnx=2x2​lnx−4x2​+C.

    Hence,

    y=x22−x24ln⁡x+Cln⁡x.y=\frac{x^2}{2}-\frac{x^2}{4\ln x}+\frac{C}{\ln x}.y=2x2​−4lnxx2​+lnxC​.

  6. Use the initial condition y(2)=2y(2)=2y(2)=2

    Substitute x=2x=2x=2:

    2=42−44ln⁡2+Cln⁡22=\frac{4}{2}-\frac{4}{4\ln 2}+\frac{C}{\ln 2}2=24​−4ln24​+ln2C​ 2=2−1ln⁡2+Cln⁡2.2=2-\frac{1}{\ln 2}+\frac{C}{\ln 2}.2=2−ln21​+ln2C​.

    Thus,

    −1ln⁡2+Cln⁡2=0-\frac{1}{\ln 2}+\frac{C}{\ln 2}=0−ln21​+ln2C​=0 C=1.C=1.C=1.

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

    Since ln⁡e=1\ln e=1lne=1,

    y(e)=e22−e24+1y(e)=\frac{e^2}{2}-\frac{e^2}{4}+1y(e)=2e2​−4e2​+1 =e24+1=\frac{e^2}{4}+1=4e2​+1 =4+e24.=\frac{4+e^2}{4}.=44+e2​.

  8. Compare with options

    4+e24\frac{4+e^2}{4}44+e2​

    This matches Option D.

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