Differential Equations
Differential Equations
Allen Star Batch
Grade 12

Question:

The general solution of the differential equation $x\left(\frac{dy}{dx}\right) = y\log\left(\frac{y}{x}\right)$ is:
$y = xe^{1-Cx}$
$y = xe^{1+Cx}$
$y = cxe^{Cx}$
$y = xe^{Cx}$

Step-by-Step Solution

Key Concept: Homogeneous equations are solved by substituting $y = vx$ to separate variables and reduce to a standard integrable form.
For the differential equation $x\frac{dy}{dx} = y\log\left(\frac{y}{x}\right)$, substitute $y = vx$ to get $v + x\frac{dv}{dx} = v\log v$. This simplifies to $\frac{dv}{v(\log v - 1)} = \frac{dx}{x}$. Integrating both sides with substitution $t = \log v$ yields $\log(t-1) = \log(xC)$, leading to $\log\frac{y}{x} = 1 + xC$ and finally $y = xe^{1+xC} = cxe^{xC}$.
Correct Answer: 2,3

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