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JEE Main 202125 JulyMorning Shift
Exam
JEE Main
Year
2021
Date
25 July
Shift
Morning Shift
Subject
Mathematics
Chapter
Differential Equations

JEE Main 2021 · 25 July · Morning 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=1+xey−x,−2<x<2,y(0)=0\dfrac{dy}{dx} = 1 + x{e}^{y - x}, - \sqrt{2} < x < \sqrt{2}, y\left(0\right) = 0, then the minimum value of y(x),x∈(−2,2)y\left(x\right), x \in \left(- \sqrt{2}, \sqrt{2}\right) is equal to :Let y = y ( x ) be the solution of the differential equation (dy)/(dx) = 1 + x e ^ y - x , - 2 < x < 2 , y(0) = 0 ,

Options

  1. A(2−3)−log⁡e2\left(2 - \sqrt{3}\right) - \log_{e}2
  2. B(2+3)+log⁡e2\left(2 + \sqrt{3}\right) + \log_{e}2
  3. C(1+3)−log⁡e(3−1)\left(1 + \sqrt{3}\right) - \log_{e}\left(\sqrt{3} - 1\right)
  4. D(1−3)−log⁡e(3−1)\left(1 - \sqrt{3}\right) - \log_{e}\left(\sqrt{3} - 1\right)

Correct answer

D. (1−3)−log⁡e(3−1)\left(1 - \sqrt{3}\right) - \log_{e}\left(\sqrt{3} - 1\right)

Step-by-step solution

Given,

dydx=1+xey−x\dfrac{dy}{dx} = 1 + x{e}^{y - x}

⇒dy−dxey−x=xdx⇒ \dfrac{dy - dx}{{e}^{y - x}} = xdx

⇒∫d(y−x)ey−x=∫xdx⇒ ∫ \dfrac{d\left(y - x\right)}{{e}^{y - x}} = ∫ xdx

⇒−ex−y=x22+c⇒ - {e}^{x - y} = \dfrac{{x}^{2}}{2} + c

At x=0,y=0⇒c=−1x = 0, y = 0 ⇒ c = - 1

So, the particular solution is

ex−y=2−x22{e}^{x - y} = \dfrac{2 - {x}^{2}}{2}

⇒y=x−ln⁡(2−x22)⇒ y = x - \ln \left(\dfrac{2 - {x}^{2}}{2}\right)

⇒dydx=1+2x2−x2=2+2x−x22−x2⇒ \dfrac{dy}{dx} = 1 + \dfrac{2x}{2 - {x}^{2}} = \dfrac{2 + 2x - {x}^{2}}{2 - {x}^{2}}

⇒dydx=x2−2x−2x2−2⇒ \dfrac{dy}{dx} = \dfrac{{x}^{2} - 2x - 2}{{x}^{2} - 2}

⇒dydx=x2−2x−2(x+2)(x−2)⇒ \dfrac{dy}{dx} = \dfrac{{x}^{2} - 2x - 2}{\left(x + \sqrt{2}\right)\left(x - \sqrt{2}\right)}

If dydx=0⇒x2−2x−2=0\dfrac{dy}{dx} = 0 ⇒ {x}^{2} - 2x - 2 = 0

⇒x=2±122⇒ x = \dfrac{2 \pm \sqrt{12}}{2}

⇒x=1±3⇒ x = 1 \pm \sqrt{3}

Let y = y ( x ) be the solution of the differential equation (dy)/(dx) = 1 + x e ^ y - x , - 2 < x < 2 , y(0) = 0 ,

So minimum value occurs at x=1−3x = 1 - \sqrt{3}

y(1−3)=(1−3)−ln⁡(2−(4−23)2)y\left(1 - \sqrt{3}\right) = \left(1 - \sqrt{3}\right) - \ln \left(\dfrac{2 - ( 4 - 2\sqrt{3} )}{2}\right)

=(1−3)−ln⁡(3−1)= \left(1 - \sqrt{3}\right) - \ln \left(\sqrt{3} - 1\right)

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