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Differential Equations question

2019 · 9 Apr · Shift 1 · Q41
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Differential Equations question

2019 · 9 Apr · Shift 1 · Q41

JEE MainMathematicsDifferential EquationsMCQ+4 / −1
The solution of the differential equation xdydx+2yx{{dy} \over {dx}} + 2yxdxdy​+2y= x2 (x eee 0) with y(1) = 1, is :
  1. A
    y=45x3+15x2y = {4 \over 5}{x^3} + {1 \over {5{x^2}}}y=54​x3+5x21​
  2. B
    y=34x2+14x2y = {3 \over 4}{x^2} + {1 \over {4{x^2}}}y=43​x2+4x21​
  3. C
    y=x24+34x2y = {{{x^2}} \over 4} + {3 \over {4{x^2}}}y=4x2​+4x23​
  4. D
    y=x35+15x2y = {{{x^3}} \over 5} + {1 \over {5{x^2}}}y=5x3​+5x21​
View written solutionFree

Correct answer: C

  1. Given differential equation

    xdydx+2y=x2,x≠0x\frac{dy}{dx} + 2y = x^2, \qquad x\neq 0xdxdy​+2y=x2,x=0

    with initial condition

    y(1)=1.y(1)=1.y(1)=1.

  2. Put it in standard linear form

    Divide the equation by xxx:

    dydx+2xy=x.\frac{dy}{dx} + \frac{2}{x}y = x.dxdy​+x2​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)=2x,Q(x)=x.P(x)=\frac{2}{x}, \qquad Q(x)=x.P(x)=x2​,Q(x)=x.

  3. Find the integrating factor (I.F.)

    I.F.=e∫P(x) dx=e∫2x dx=e2ln⁡∣x∣=x2\text{I.F.} = e^{\int P(x)\,dx} = e^{\int \frac{2}{x}\,dx} = e^{2\ln|x|} = x^2I.F.=e∫P(x)dx=e∫x2​dx=e2ln∣x∣=x2

    (since x≠0x\neq 0x=0, we can take x2x^2x2 as the integrating factor).

  4. Multiply the differential equation by the integrating factor

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

    The left-hand side is:

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

  5. Integrate both sides

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

    Therefore,

    y=x44x2+Cx2=x24+Cx2.y = \frac{x^4}{4x^2} + \frac{C}{x^2} = \frac{x^2}{4} + \frac{C}{x^2}.y=4x2x4​+x2C​=4x2​+x2C​.

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

    Substitute x=1x=1x=1 and y=1y=1y=1:

    1=124+C12=14+C.1 = \frac{1^2}{4} + \frac{C}{1^2} = \frac{1}{4} + C.1=412​+12C​=41​+C.

    So,

    C=1−14=34.C = 1 - \frac{1}{4} = \frac{3}{4}.C=1−41​=43​.

  7. Final solution

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

  8. Compare with options

    • A: 45x3+15x2\frac{4}{5}x^3 + \frac{1}{5x^2}54​x3+5x21​ — incorrect
    • B: 34x2+14x2\frac{3}{4}x^2 + \frac{1}{4x^2}43​x2+4x21​ — incorrect
    • C: x24+34x2\frac{x^2}{4} + \frac{3}{4x^2}4x2​+4x23​ — correct
    • D: x35+15x2\frac{x^3}{5} + \frac{1}{5x^2}5x3​+5x21​ — incorrect

Hence, the correct option is C.

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