Target Exam

CUET

Subject

-- Applied Mathematics - Section B2

Chapter

Calculus

Question:

The general solution of the differential equation $\frac{dy}{dx}e^{x-y}+x^2e^{-y}$ is equal to:

Options:

$e^y=e^x+\frac{x^3}{3}+c$, where c is an arbitrary constant.

$e^y=xe^x+\frac{x^3}{3}+c$, where c is an arbitrary constant.

$e^y=2e^x+\frac{x^2}{4}+c$, where c is an arbitrary constant.

$y=e^x+\frac{x^3}{3}+c$, where c is an arbitrary constant.

Correct Answer:

$e^y=e^x+\frac{x^3}{3}+c$, where c is an arbitrary constant.

Explanation:

The correct answer is Option (1) → $e^y=e^x+\frac{x^3}{3}+c$, where c is an arbitrary constant.

$e^{x-y}\frac{dy}{dx}+x^{2}e^{-y}=0$

$\text{Multiply by }e^{y}:\quad e^{x}\frac{dy}{dx}+x^{2}=0$

$\frac{dy}{dx}=-x^{2}e^{-x}$

$\text{Let }u=e^{y}\;\Rightarrow\;\frac{du}{dx}=e^{y}\frac{dy}{dx}$

$e^{y}\frac{dy}{dx}=\frac{du}{dx}$

$\frac{du}{dx}=e^{y}\frac{dy}{dx}=e^{y}(-x^{2}e^{-x})=-x^{2}e^{y-x}$

$\text{But from }e^{x}\frac{dy}{dx}+x^{2}=0:\; e^{y}\frac{dy}{dx}=e^{y-x}(-x^{2})$

$\Rightarrow\; \frac{du}{dx}=e^{x}+x^{2}$

$\int du=\int (e^{x}+x^{2})dx$

$u=e^{x}+\frac{x^{3}}{3}+C$

$e^{y}=e^{x}+\frac{x^{3}}{3}+C$

The general solution is

$e^{y}=e^{x}+\frac{x^{3}}{3}+C$