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JEE Main 202127 JulyMorning Shift
Exam
JEE Main
Year
2021
Date
27 July
Shift
Morning Shift
Subject
Mathematics
Chapter
Differential Equations

JEE Main 2021 · 27 July · Morning Shift · Mathematics · Differential Equations

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

Let y=yy = y\left(x\right)besolutionofthedifferentialequationbe solution of the differential equation\log_{e}\left(\dfrac{dy}{dx}\right) = 3x + 4y,withwithy\left(0\right) = 0.If. Ify\left(- \dfrac{2}{3}\log_{e}2\right) = \alpha{\log}_{e}2,thenthevalueof, then the value of\alpha is equal to:

Options

  1. A−14- \dfrac{1}{4}
  2. B14\dfrac{1}{4}
  3. C22
  4. D−12- \dfrac{1}{2}

Correct answer

A. −14- \dfrac{1}{4}

Step-by-step solution

Given,

log⁡e(dydx)=3x+4y\log_{e}\left(\dfrac{dy}{dx}\right) = 3x + 4y

⇒dydx=e3x⋅e4y⇒ \dfrac{dy}{dx} = {e}^{3x} \cdot {e}^{4y}

⇒∫e−4ydy=∫e3xdx⇒ ∫ {e}^{- 4y}dy = ∫ {e}^{3x}dx

⇒e−4y−4=e3x3+C⇒ \dfrac{{e}^{- 4y}}{- 4} = \dfrac{{e}^{3x}}{3} + C

Given,

y(0)=0y\left(0\right) = 0

So,

−14−13=C⇒C=−712- \dfrac{1}{4} - \dfrac{1}{3} = C ⇒ C = - \dfrac{7}{12}

So, the particular solution is

e−4y−4=e3x3−712\dfrac{{e}^{- 4y}}{- 4} = \dfrac{{e}^{3x}}{3} - \dfrac{7}{12}

⇒e−4y=4e3x−7−3⇒ {e}^{- 4y} = \dfrac{4{e}^{3x} - 7}{- 3}

⇒e4y=37−4e3x⇒4y=ln⁡(37−4e3x)⇒ {e}^{4y} = \dfrac{3}{7 - 4{e}^{3x}} ⇒ 4y = \ln \left(\dfrac{3}{7 - 4{e}^{3x}}\right)

4y=ℓn(36)4y = ℓn\left(\dfrac{3}{6}\right) when x=−23ℓn2x = - \dfrac{2}{3}ℓn2

⇒y=14ℓn(12)⇒ y = \dfrac{1}{4}ℓn\left(\dfrac{1}{2}\right)

⇒y=−14ℓn2⇒ y = - \dfrac{1}{4}ℓn2

So, α=−14\alpha = - \dfrac{1}{4}

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