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BITSAT 2018
Exam
BITSAT
Year
2018
Subject
Mathematics
Chapter
Differential Equations

BITSAT 2018 · Mathematics · Differential Equations

The equation of the curve whose slope at any point is equal to y +...

The equation of the curve whose slope at any point is equal to y+2xy + 2x and which passes through the origin is

Options

  1. Ay=2(x−1)y = 2 \left(x - 1\right)
  2. By=2(ex−x−1)y = 2 \left(e^{x} - x - 1\right)
  3. Cy=2(ex−1)y = 2 \left(e^{x} - 1\right)
  4. Dy=2(exx−1)y = 2 \left(e^{x}x - 1\right)

Correct answer

B. y=2(ex−x−1)y = 2 \left(e^{x} - x - 1\right)

Step-by-step solution

We have, dydx=y+2x\dfrac{dy}{dx} = y + 2x

⇒dydx−y=2x⇒ \dfrac{dy}{dx} - y = 2x

Now, IF=e−∫1dx=e−xIF = e^{- ∫ 1dx} = e^{- x}

y⋅e−x=∫2xe−xdx+k,ky \cdot e^{- x} = ∫ 2xe^{- x}dx + k, k be the constant of integration

=2[x∫e−xdx−∫1⋅(−e−x)dx]+k= 2\left[x ∫ e^{- x}dx - ∫ 1 \cdot \left(- e^{- x}\right)dx\right] + k [using integration by parts]

⇒y⋅e−x=−2xe−x−2e−x+k...(i)⇒ y \cdot e^{- x} = - 2xe^{- x} - 2e^{- x} + k... \left(i\right)

As curve (i) passes through \left(0, 0\right)</p><p></p><p>∴ 0 = 0 - 2 + k</p><p></p><p>⇒ k = 2</p><p>Thus,thecurveis</p><p></p><p>Thus, the curve is</p><p>ye^{- x} = - 2xe^{- x} - 2e^{- x} + 2</p><p></p><p>∴ y = 2 \left(e^{x} - x - 1\right)

Hence, option (b) is correct.

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