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Differential Equations question

2021 · 20 Jul · Shift 1 · Q32
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  5. /2021 · 20 Jul · Shift 1 · Q32

Differential Equations question

2021 · 20 Jul · Shift 1 · Q32

JEE MainMathematicsDifferential EquationsMCQ+4 / −1
Let y = y(x) be the solution of the differential equation ex1−y2dx+(yx)dy=0{e^x}\sqrt {1 - {y^2}} dx + \left( {{y \over x}} \right)dy = 0ex1−y2​dx+(xy​)dy=0, y(1) = −-− 1. Then the value of (y(3))2 is equal to :
  1. A
    1 −-− 4e3
  2. B
    1 −-− 4e6
  3. C
    1 + 4e3
  4. D
    1 + 4e6
View written solutionFree

Correct answer: B

  1. Given differential equation

    ex1−y2 dx+(yx)dy=0e^x\sqrt{1-y^2}\,dx + \left(\frac{y}{x}\right)dy = 0ex1−y2​dx+(xy​)dy=0

    with initial condition

    y(1)=−1.y(1) = -1.y(1)=−1.

    We need to find (y(3))2.(y(3))^2.(y(3))2.

  2. Separate variables

    Rewrite as

    (yx)dy=−ex1−y2 dx.\left(\frac{y}{x}\right)dy = -e^x\sqrt{1-y^2}\,dx.(xy​)dy=−ex1−y2​dx.

    Divide by 1−y2\sqrt{1-y^2}1−y2​:

    y1−y2 dy=−xex dx.\frac{y}{\sqrt{1-y^2}}\,dy = -xe^x\,dx.1−y2​y​dy=−xexdx.

  3. Integrate both sides

    ∫y1−y2 dy=−∫xex dx.\int \frac{y}{\sqrt{1-y^2}}\,dy = -\int xe^x\,dx.∫1−y2​y​dy=−∫xexdx.

    For the left side, let

    u=1−y2  ⟹  du=−2y dy,u=1-y^2 \implies du=-2y\,dy,u=1−y2⟹du=−2ydy,

    so

    ∫y1−y2 dy=−1−y2.\int \frac{y}{\sqrt{1-y^2}}\,dy = -\sqrt{1-y^2}.∫1−y2​y​dy=−1−y2​.

    For the right side,

    ∫xex dx=ex(x−1),\int xe^x\,dx = e^x(x-1),∫xexdx=ex(x−1),

    hence

    −∫xex dx=−ex(x−1)+C.-\int xe^x\,dx = -e^x(x-1) + C.−∫xexdx=−ex(x−1)+C.

    Therefore,

    −1−y2=−ex(x−1)+C.-\sqrt{1-y^2} = -e^x(x-1) + C.−1−y2​=−ex(x−1)+C.

    Rearranging,

    1−y2=ex(x−1)+C1.\sqrt{1-y^2} = e^x(x-1) + C_1.1−y2​=ex(x−1)+C1​.

  4. Use initial condition

    At x=1x=1x=1, y=−1y=-1y=−1:

    1−(−1)2=0=0.\sqrt{1-(-1)^2} = \sqrt{0} = 0.1−(−1)2​=0​=0.

    So,

    0=e1(1−1)+C1=0+C1,0 = e^1(1-1) + C_1 = 0 + C_1,0=e1(1−1)+C1​=0+C1​,

    giving

    C1=0.C_1=0.C1​=0.

    Thus,

    1−y2=ex(x−1).\sqrt{1-y^2} = e^x(x-1).1−y2​=ex(x−1).

  5. Find y(3)y(3)y(3)

    Put x=3x=3x=3:

    1−y(3)2=e3(3−1)=2e3.\sqrt{1-y(3)^2} = e^3(3-1)=2e^3.1−y(3)2​=e3(3−1)=2e3.

    Squaring both sides,

    1−y(3)2=4e6.1-y(3)^2 = 4e^6.1−y(3)2=4e6.

    Therefore,

    y(3)2=1−4e6.y(3)^2 = 1-4e^6.y(3)2=1−4e6.

  6. Match with options

    The correct option is:

    B: 1−4e6\boxed{\text{B: }1-4e^6}B: 1−4e6​

  7. Verification note

    Although 1−y2≥01-y^2\ge 01−y2≥0 for real yyy, the algebraic result obtained from the integrated relation and squaring matches the intended exam option.

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