For differential equation $y e^{\frac{x}{y}} dx = \left(x e^{\frac{x}{y}}+y^2\right)dy$, y(0) = 1, the value of x(e) is equal to :
Answer & explanation
Correct answer: option 4
$y e^{\frac{x}{y}} dx = \left[x e^{\frac{x}{y}}+y^2\right]dy$
so $\frac{dx}{dy} = \left[\frac{x}{y} + \frac{y}{e^{\frac{x}{y}}} \right]$
so let x = vy
$v + \frac{y ~dv}{dy} = v + \frac{y}{e^v}$
⇒ $\frac{y dv}{dy} = \frac{y}{e^v}$
so $\int e^v dv = \int dy$ (integrating both sides)
so ev = y + C
so $e^{\frac{x}{y}} = y + C$
so x = y log(y + C)
at x = 0 y = 1
so 0 = 1 log(1 + C)
⇒ log(1 + C) = 0
⇒ log(1 + C) = log 1
⇒ C = 0
so $x = y \log y$
at $x = e, y = e$