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JEE Advanced 2021Paper 2
Exam
JEE Advanced
Year
2021
Paper
Paper 2
Subject
Mathematics
Chapter
Differential Equations

JEE Advanced 2021 · Paper 2 · Mathematics · Differential Equations

For any real numbers α and β , let y _ α, β (x), x ∈ R , be the...

For any real numbers α\alpha and β\beta, let yα,β(x),x∈R{y}_{\alpha, \beta}\left(x\right), x \in R, be the solution of the differential equation

dydx+\alphay=xe\betax,y(1)=1\dfrac{dy}{dx} + \alphay = x{e}^{\betax}, y\left(1\right) = 1

Let S={yα,β(x):α,β∈R}S = \left\{{y}_{\alpha, \beta}\left(x\right): \alpha, \beta \in R\right\}. Then which of the following functions belong(s) to the set SS?

Options

  1. Af(x)=x22e−x+(e−12)e−xf\left(x\right) = \dfrac{{x}^{2}}{2}{e}^{- x} + \left(e - \dfrac{1}{2}\right){e}^{- x}
  2. Bf(x)=−x22e−x+(e+12)e−xf\left(x\right) = - \dfrac{{x}^{2}}{2}{e}^{- x} + \left(e + \dfrac{1}{2}\right){e}^{- x}
  3. Cf(x)=ex2(x−12)+(e−e24)e−xf\left(x\right) = \dfrac{{e}^{x}}{2}\left(x - \dfrac{1}{2}\right) + \left(e - \dfrac{{e}^{2}}{4}\right){e}^{- x}
  4. Df(x)=ex2(12−x)+(e+e24)e−xf\left(x\right) = \dfrac{{e}^{x}}{2}\left(\dfrac{1}{2} - x\right) + \left(e + \dfrac{{e}^{2}}{4}\right){e}^{- x}

Correct answer

A. f(x)=x22e−x+(e−12)e−xf\left(x\right) = \dfrac{{x}^{2}}{2}{e}^{- x} + \left(e - \dfrac{1}{2}\right){e}^{- x}

Step-by-step solution

(\frac{d y}{d x}+a y=x e^{\beta x}) Integrating factor (I.F.) (=\mathrm{e}^{\mathrm{adx}}=\mathrm{e}^{\mathrm{ax}}) So, the solution is (y \cdot e^{a x}=\int x e^{\beta x} \cdot e^{a x} d x) (\Rightarrow y e^{a x}=\int x e^{(\alpha+\beta) x} d x) If (\alpha+\beta \neq 0) (\Rightarrow y e^{a x}=x \frac{e^{(\alpha+\beta) x}}{(\alpha+\beta)}-\frac{e^{(\alpha+\beta) x}}{(\alpha+\beta)^2}+C) (\Rightarrow y=\frac{x e^{\beta x}}{(\alpha+\beta)}-\frac{e^{\beta x}}{(\alpha+\beta)^2}+C e^{-a x}) (\Rightarrow y=\frac{e^{\beta x}}{(\alpha+\beta)}\left(x-\frac{1}{a+\beta}\right)+C e^{-\alpha x}) Put (\alpha=\beta=1) in (1) (y=\frac{e^x}{2}\left(x-\frac{1}{2}\right)+C e^{-x}) y(1)=1y(1)=1 (1=\frac{e}{2} \times \frac{1}{2}+\frac{C}{e} \Rightarrow c=e-\frac{e^2}{4}) So, (y=\frac{e^x}{2}\left(x-\frac{1}{2}\right)+\left(e-\frac{e^2}{4}\right) e^{-x}) If (\alpha+\beta=0 \& \alpha=1) dydx+y=xe−x I.F. =ex\begin{aligned} &\frac{d y}{d x}+y=x e^{-x} \\ &\text{ I.F. }=e^x \end{aligned}) Solution is (y e^x=\int x d x) (\Rightarrow y e^x=\frac{x^2}{2}+C) (y=\frac{x^2}{2} e^{-x}+C e^{-x}) y(1)=1y(1)=1 (1=\frac{1}{2 e}+\frac{C}{e} \Rightarrow C=e-\frac{1}{2}) (y=\frac{x^2}{2} e^{-x}+\left(e-\frac{1}{2}\right) e^{-x})$
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