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Differential Equations question

2019 · 9 Jan · Shift 1 · Q30
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  5. /2019 · 9 Jan · Shift 1 · Q30

Differential Equations question

2019 · 9 Jan · Shift 1 · Q30

JEE MainMathematicsDifferential EquationsMCQ+4 / −1
If y = y(x) is the solution of the differential equation, x dydxdy \over dxdxdy​ + 2y = x2, satisfying y(1) = 1, then y(121\over221​) is equal to :
  1. A
    764{{7} \over {64}}647​
  2. B
    4916{{49} \over {16}}1649​
  3. C
    14{{1} \over {4}}41​
  4. D
    1316{{13} \over {16}}1613​
View written solutionFree

Correct answer: B

  1. Given differential equation

    xdydx+2y=x2,y(1)=1x\frac{dy}{dx}+2y=x^2, \qquad y(1)=1xdxdy​+2y=x2,y(1)=1

    We first write it in standard linear form by dividing by xxx (for x≠0x\neq 0x=0):

    dydx+2xy=x\frac{dy}{dx}+\frac{2}{x}y=xdxdy​+x2​y=x

  2. Identify integrating factor

    For the linear differential equation

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

    here

    P(x)=2xP(x)=\frac{2}{x}P(x)=x2​

    So the integrating factor is

    I.F.=e∫2x dx=e2ln⁡x=x2\text{I.F.}=e^{\int \frac{2}{x}\,dx}=e^{2\ln x}=x^2I.F.=e∫x2​dx=e2lnx=x2

    (taking x>0x>0x>0 since the condition is given at x=1x=1x=1 and we need x=12x=\tfrac12x=21​).

  3. Multiply throughout by the integrating factor

    x2dydx+2xy=x3x^2\frac{dy}{dx}+2xy=x^3x2dxdy​+2xy=x3

    The left-hand side is the derivative of x2yx^2yx2y:

    ddx(x2y)=x3\frac{d}{dx}(x^2y)=x^3dxd​(x2y)=x3

  4. Integrate both sides

    x2y=∫x3 dx=x44+Cx^2y=\int x^3\,dx=\frac{x^4}{4}+Cx2y=∫x3dx=4x4​+C

    Hence,

    y=x24+Cx2y=\frac{x^2}{4}+\frac{C}{x^2}y=4x2​+x2C​

  5. Use the initial condition y(1)=1y(1)=1y(1)=1

    1=14+C1=\frac{1}{4}+C1=41​+C

    So,

    C=34C=\frac{3}{4}C=43​

    Therefore,

    y=x24+34x2y=\frac{x^2}{4}+\frac{3}{4x^2}y=4x2​+4x23​

  6. Find y(12)y\left(\frac12\right)y(21​)

    y(12)=(1/2)24+34(1/2)2y\left(\frac12\right)=\frac{(1/2)^2}{4}+\frac{3}{4(1/2)^2}y(21​)=4(1/2)2​+4(1/2)23​

    =1/44+34⋅1/4=\frac{1/4}{4}+\frac{3}{4\cdot 1/4}=41/4​+4⋅1/43​

    =116+3=\frac{1}{16}+3=161​+3

    =116+4816=4916=\frac{1}{16}+\frac{48}{16}=\frac{49}{16}=161​+1648​=1649​

  7. Check options

    4916\frac{49}{16}1649​

    matches Option B.

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