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JEE Main 202431 JanuaryMorning Shift
Exam
JEE Main
Year
2024
Date
31 January
Shift
Morning Shift
Subject
Mathematics
Chapter
Differential Equations

JEE Main 2024 · 31 January · Morning Shift · Mathematics · Differential Equations

The solution curve of the differential equation y(dx)/(dy) = x( _ e...

The solution curve of the differential equation ydxdy=x(log⁡ex−log⁡ey+1),x>0,y>0y\dfrac{dx}{dy} = x\left(\log_{e}x - \log_{e}y + 1\right), x > 0, y > 0 passing through the point (e,1)( e, 1 ) is

Options

  1. A∣log⁡eyx∣=x\left|\log_{e}\dfrac{y}{x}\right| = x
  2. B∣log⁡eyx∣=y2\left|\log_{e}\dfrac{y}{x}\right| = {y}^{2}
  3. C∣log⁡exy∣=y\left|\log_{e}\dfrac{x}{y}\right| = y
  4. D2∣log⁡exy∣=y+12\left|\log_{e}\dfrac{x}{y}\right| = y + 1

Correct answer

C. ∣log⁡exy∣=y\left|\log_{e}\dfrac{x}{y}\right| = y

Step-by-step solution

Given: ydxdy=x(log⁡ex−log⁡ey+1),x>0,y>0y\dfrac{dx}{dy} = x\left(\log_{e}x - \log_{e}y + 1\right), x > 0, y > 0

⇒dxdy=(xy)[log⁡e(xy)+1]⇒ \dfrac{dx}{dy} = \left(\dfrac{x}{y}\right)\left[\log_{e}\left(\dfrac{x}{y}\right) + 1\right]

Putting, x=vyx = vy

⇒dxdy=v+ydvdx⇒ \dfrac{dx}{dy} = v + y\dfrac{dv}{dx}

⇒v+ydvdy=(vyy)[log⁡e(vyy)+1]⇒ v + y\dfrac{dv}{dy} = \left(\dfrac{vy}{y}\right)\left[\log_{e}\left(\dfrac{vy}{y}\right) + 1\right]

⇒v+ydvdy=v[log⁡e(v)+1]⇒ v + y\dfrac{dv}{dy} = v\left[\log_{e}\left(v\right) + 1\right]

⇒v+ydvdy=vlog⁡e(v)+v⇒ v + y\dfrac{dv}{dy} = v{\log}_{e}\left(v\right) + v

⇒ydvdy=vlog⁡e(v)⇒ y\dfrac{dv}{dy} = v{\log}_{e}\left(v\right)

⇒dvvlog⁡ev=dyy⇒ \dfrac{dv}{v{\log}_{e}v} = \dfrac{dy}{y}

⇒∫dvvlog⁡ev=∫dyy⇒ ∫ \dfrac{dv}{v{\log}_{e}v} = ∫ \dfrac{dy}{y}

Putting, log⁡ev=t\log_{e}v = t

⇒dvv=dt⇒ \dfrac{dv}{v} = dt

⇒∫dtt=∫dyy⇒ ∫ \dfrac{dt}{t} = ∫ \dfrac{dy}{y}

⇒log⁡t=log⁡y+c⇒ \log t = \log y + c

⇒log⁡(log⁡exy)=log⁡y+c⇒ \log \left(\log_{e}\dfrac{x}{y}\right) = \log y + c

Using point \left(e, 1\right)</p><p></p><p>⇒ \log \left(\log_{e}e\right) = \log \left(1\right) + c</p><p></p><p>⇒ c = 0</p><p></p><p>⇒ \log \left(\log_{e}\dfrac{x}{y}\right) = \log y</p><p></p><p>⇒ \left|\log_{e}\dfrac{x}{y}\right| = y$

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