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JEE Main 20211 SeptemberEvening Shift
Exam
JEE Main
Year
2021
Date
1 September
Shift
Evening Shift
Subject
Mathematics
Chapter
Differential Equations

JEE Main 2021 · 1 September · Evening Shift · Mathematics · Differential Equations

If y = y ( x ) is the solution curve of the differential equation x...

If y=y(x)y = y ( x ) is the solution curve of the differential equation x2dy+(y−1x)dx=0;x>0{x}^{2}dy + \left(y - \dfrac{1}{x}\right)dx = 0; x > 0 and y(1)=1,y ( 1 ) = 1, then y(12)y\left(\dfrac{1}{2}\right) is equal to :

Options

  1. A3+e3 + e
  2. B3−e3 - e
  3. C32−1e\dfrac{3}{2} - \dfrac{1}{\sqrt{e}}
  4. D3+1e3 + \dfrac{1}{\sqrt{e}}

Correct answer

B. 3−e3 - e

Step-by-step solution

x2dy+ydx=dxx{x}^{2}dy + ydx = \dfrac{dx}{x}

⇒dydx+yx2=1x3⇒ \dfrac{dy}{dx} + \dfrac{y}{{x}^{2}} = \dfrac{1}{{x}^{3}}

I.F=e∫1x2dx=e−1xI . F = {e}^{∫ \dfrac{1}{{x}^{2}}dx} = {e}^{- \dfrac{1}{x}}

⇒y⋅e−1x=∫e−1x⋅1x3dx+C⇒ y \cdot {e}^{- \dfrac{1}{x}} = ∫ {e}^{- \dfrac{1}{x}} \cdot \dfrac{1}{{x}^{3}}dx + C

Let−1x=t⇒1x2dx=dt\text{Let}\dfrac{- 1}{x} = t ⇒ \dfrac{1}{{x}^{2}}dx = dt

⇒y⋅e−1x=∫−tet⋅dt+C⇒ y \cdot {e}^{- \dfrac{1}{x}} = ∫ - t{e}^{t} \cdot dt + C

=−[tet−et]+C= - \left[t{e}^{t} - {e}^{t}\right] + C

⇒y⋅e−1x=1xe−1x+e−1x+C⇒ y \cdot {e}^{- \dfrac{1}{x}} = \dfrac{1}{x}{e}^{- \dfrac{1}{x}} + {e}^{- \dfrac{1}{x}} + C

Putx=1\text{Put}x = 1

⇒(1)⋅e−1=e−11+e−1+C⇒ ( 1 ) \cdot {e}^{- 1} = \dfrac{{e}^{- 1}}{1} + {e}^{- 1} + C

⇒C=−e−1⇒ C = - {e}^{- 1}

Equation is y⋅e−1x=1xe−1x+e−1x−e−1y \cdot {e}^{- \dfrac{1}{x}} = \dfrac{1}{x}{e}^{- \dfrac{1}{x}} + {e}^{- \dfrac{1}{x}} - {e}^{- 1}

⇒y=1x+1−e1xe⇒ y = \dfrac{1}{x} + 1 - \dfrac{e\dfrac{1}{x}}{e}

Atx=12⇒y(12)=2+1−e2e⇒y=3−e\text{At}x = \dfrac{1}{2} ⇒ y\left(\dfrac{1}{2}\right) = 2 + 1 - \dfrac{{e}^{2}}{e} ⇒ y = 3 - e

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