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JEE Main 202118 MarchEvening Shift
Exam
JEE Main
Year
2021
Date
18 March
Shift
Evening Shift
Subject
Mathematics
Chapter
Differential Equations

JEE Main 2021 · 18 March · Evening Shift · Mathematics · Differential Equations

Let y = y ( x ) be the solution of the differential equation...

Let y=y(x)y = y ( x ) be the solution of the differential equation dydx=(y+1)((y+1)ex2/2−x),0<x<2.1\dfrac{dy}{dx} = \left(y + 1\right)\left(\left(y + 1\right){e}^{{x}^{2} / 2} - x\right), 0 < x < 2 . 1, with y(2)=0y ( 2 ) = 0. Then the value of dydx\dfrac{dy}{dx} at x=1x = 1 is equal to

Options

  1. A\dfrac{- {e}^{3 / 2}}{{\left({e}^{2} + 1\right)}^{2}}
  2. B- \dfrac{2{e}^{2}}{{\left(1 + {e}^{2}\right)}^{2}}
  3. C\dfrac{{e}^{5 / 2}}{{\left(1 + {e}^{2}\right)}^{2}}
  4. D\dfrac{5{e}^{1 / 2}}{{\left({e}^{2} + 1\right)}^{2}}

Correct answer

A. \dfrac{- {e}^{3 / 2}}{{\left({e}^{2} + 1\right)}^{2}}

Step-by-step solution

Let y+1=Yy + 1 = Y

∴dYdx=Y2ex22−xY∴ \dfrac{dY}{dx} = {Y}^{2}e\dfrac{{x}^{2}}{2} - xY

Put −1Y=k- \dfrac{1}{Y} = k

⇒dkdx+k(−x)=ex22⇒ \dfrac{dk}{dx} + k\left(- x\right) = e\dfrac{{x}^{2}}{2}

I.F.=e−x22I . F . = {e}^{- \dfrac{{x}^{2}}{2}}

∴k=(x+c)ex2/2∴ k = \left(x + c\right){e}^{{x}^{2} / 2}

Put k=−1y+1k = - \dfrac{1}{y + 1}

∴y+1=−1(x+c)ex2/2...(i)∴ y + 1 = - \dfrac{1}{( x + c ) {e}^{{x}^{2} / 2}} . . . \left(i\right)

when x=2,y=0x = 2, y = 0, then c=−2−1e2c = - 2 - \dfrac{1}{{e}^{2}}

Differentiate equation (i) and ( i ) \text{ and } put x=1x = 1 we get,

(dydx)x=1=−e3/2(1+e2)2{\left(\dfrac{dy}{dx}\right)}_{x = 1} = - \dfrac{{e}^{3 / 2}}{{\left(1 + {e}^{2}\right)}^{2}}

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