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Differential Equations question

2015 · Shift 0 · Q28
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  5. /2015 · Shift 0 · Q28

Differential Equations question

2015 · Shift 0 · Q28

JEE MainMathematicsDifferential EquationsMCQ+4 / −1
Let y(x)y(x)y(x) be the solution of the differential equation (x log⁡x)dydx+y=2x log⁡x,(x≥1).\left( {x\,\log x} \right){{dy} \over {dx}} + y = 2x\,\log x,\left( {x \ge 1} \right).(xlogx)dxdy​+y=2xlogx,(x≥1). Then y(e)y(e)y(e) is equal to :
  1. A
    222
  2. B
    2e2e2e
  3. C
    eee
  4. D
    000
View written solutionFree

Correct answer: A

  1. Given differential equation

    (xlog⁡x)dydx+y=2xlog⁡x,x≥1(x\log x)\frac{dy}{dx} + y = 2x\log x, \qquad x\ge 1(xlogx)dxdy​+y=2xlogx,x≥1

    We need to find y(e)y(e)y(e).

  2. Rewrite in standard linear form

    Divide both sides by xlog⁡xx\log xxlogx (for x>1x>1x>1):

    dydx+1xlog⁡xy=2\frac{dy}{dx} + \frac{1}{x\log x}y = 2dxdy​+xlogx1​y=2

    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)=1xlog⁡x,Q(x)=2.P(x)=\frac{1}{x\log x}, \qquad Q(x)=2.P(x)=xlogx1​,Q(x)=2.

  3. Find the integrating factor

    IF=e∫P(x) dx=e∫1xlog⁡x dxIF = e^{\int P(x)\,dx} = e^{\int \frac{1}{x\log x}\,dx}IF=e∫P(x)dx=e∫xlogx1​dx

    Now,

    ∫1xlog⁡x dx=log⁡(log⁡x)\int \frac{1}{x\log x}\,dx = \log(\log x)∫xlogx1​dx=log(logx)

    so

    IF=elog⁡(log⁡x)=log⁡xIF = e^{\log(\log x)} = \log xIF=elog(logx)=logx

    (since x>1x>1x>1, log⁡x>0\log x>0logx>0).

  4. Multiply the equation by the integrating factor

    (log⁡x)dydx+yx=2log⁡x(\log x)\frac{dy}{dx} + \frac{y}{x} = 2\log x(logx)dxdy​+xy​=2logx

    Observe that the left side is:

    ddx(ylog⁡x)\frac{d}{dx}(y\log x)dxd​(ylogx)

    because

    ddx(ylog⁡x)=log⁡xdydx+y⋅1x.\frac{d}{dx}(y\log x)=\log x\frac{dy}{dx}+y\cdot\frac{1}{x}.dxd​(ylogx)=logxdxdy​+y⋅x1​.

    Hence,

    ddx(ylog⁡x)=2log⁡x.\frac{d}{dx}(y\log x)=2\log x.dxd​(ylogx)=2logx.

  5. Integrate both sides

    ylog⁡x=∫2log⁡x dx+Cy\log x = \int 2\log x\,dx + Cylogx=∫2logxdx+C

    Using

    ∫log⁡x dx=xlog⁡x−x,\int \log x\,dx = x\log x - x,∫logxdx=xlogx−x,

    we get

    ylog⁡x=2(xlog⁡x−x)+C.y\log x = 2(x\log x - x) + C.ylogx=2(xlogx−x)+C.

    Therefore,

    y=2xlog⁡x−2x+Clog⁡x.y = \frac{2x\log x - 2x + C}{\log x}.y=logx2xlogx−2x+C​.

  6. Use the condition x≥1x\ge 1x≥1 to determine the solution

    Since the solution is given for x≥1x\ge 1x≥1, it must be defined at x=1x=1x=1 as well. But log⁡1=0\log 1 = 0log1=0, so for yyy to remain finite at x=1x=1x=1, the numerator must also vanish at x=1x=1x=1.

    At x=1x=1x=1:

    2(1)log⁡1−2(1)+C=0−2+C=02(1)\log 1 - 2(1) + C = 0 - 2 + C = 02(1)log1−2(1)+C=0−2+C=0

    so

    C=2.C=2.C=2.

    Thus,

    y=2xlog⁡x−2x+2log⁡x.y = \frac{2x\log x - 2x + 2}{\log x}.y=logx2xlogx−2x+2​.

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

    Since log⁡e=1\log e = 1loge=1,

    y(e)=2e(1)−2e+21=2.y(e)=\frac{2e(1)-2e+2}{1}=2.y(e)=12e(1)−2e+2​=2.

  8. Check options

    • A: 222 ✓
    • B: 2e2e2e ✗
    • C: eee ✗
    • D: 000 ✗

Therefore, the correct answer is A.

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