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Differential Equations question

2023 · 29 Jan · Shift 1 · Q32
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  5. /2023 · 29 Jan · Shift 1 · Q32

Differential Equations question

2023 · 29 Jan · Shift 1 · Q32

JEE MainMathematicsDifferential EquationsMCQ+4 / −1
Let y=f(x)y=f(x)y=f(x) be the solution of the differential equation y(x+1)dx−x2dy=0,y(1)=ey(x+1)dx-x^2dy=0,y(1)=ey(x+1)dx−x2dy=0,y(1)=e. Then lim⁡x→0+f(x)\mathop {\lim }\limits_{x \to {0^ + }} f(x)x→0+lim​f(x) is equal to
  1. A
    e2{e^2}e2
  2. B
    0
  3. C
    1e2{1 \over {{e^2}}}e21​
  4. D
    1e{1 \over e}e1​
View written solutionFree

Correct answer: B

  1. Given differential equation

    y(x+1) dx−x2 dy=0y(x+1)\,dx - x^2\,dy = 0y(x+1)dx−x2dy=0

    with initial condition

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

  2. Rewrite in differential form

    From

    y(x+1) dx=x2 dy,y(x+1)\,dx = x^2\,dy,y(x+1)dx=x2dy,

    we get

    dydx=x+1x2y.\frac{dy}{dx} = \frac{x+1}{x^2}y.dxdy​=x2x+1​y.

    So,

    1ydydx=x+1x2=1x+1x2.\frac{1}{y}\frac{dy}{dx} = \frac{x+1}{x^2} = \frac{1}{x} + \frac{1}{x^2}. y1​dxdy​=x2x+1​=x1​+x21​.

  3. Separate variables and integrate

    dyy=(1x+1x2)dx.\frac{dy}{y} = \left(\frac{1}{x} + \frac{1}{x^2}\right)dx.ydy​=(x1​+x21​)dx.

    Integrating,

    ∫dyy=∫(1x+1x2)dx,\int \frac{dy}{y} = \int \left(\frac{1}{x} + \frac{1}{x^2}\right)dx,∫ydy​=∫(x1​+x21​)dx,

    ln⁡∣y∣=ln⁡∣x∣−1x+C.\ln |y| = \ln |x| - \frac{1}{x} + C.ln∣y∣=ln∣x∣−x1​+C.

    Therefore,

    y=Cxe−1/x.y = Cx e^{-1/x}.y=Cxe−1/x.

  4. Use the initial condition

    Since y(1)=ey(1)=ey(1)=e,

    e=C(1)e−1=Ce.e = C(1)e^{-1} = \frac{C}{e}.e=C(1)e−1=eC​.

    Hence,

    C=e2.C = e^2.C=e2.

    So the solution is

    f(x)=e2xe−1/x.f(x)= e^2 x e^{-1/x}.f(x)=e2xe−1/x.

  5. Evaluate the limit as x→0+x\to 0^+x→0+

    We need

    lim⁡x→0+e2xe−1/x.\lim_{x\to 0^+} e^2 x e^{-1/x}.limx→0+​e2xe−1/x.

    As x→0+x\to 0^+x→0+, we have 1x→+∞\frac{1}{x}\to +\inftyx1​→+∞, so

    e−1/x→0e^{-1/x}\to 0e−1/x→0

    extremely fast. Therefore,

    xe−1/x→0.x e^{-1/x} \to 0.xe−1/x→0.

    Hence,

    lim⁡x→0+f(x)=0.\lim_{x\to 0^+} f(x)=0.limx→0+​f(x)=0.

  6. Check options

    • A: e2e^2e2 ❌
    • B: 000 ✅
    • C: 1e2\dfrac{1}{e^2}e21​ ❌
    • D: 1e\dfrac{1}{e}e1​ ❌

Therefore, the correct option is B.

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