Question

dy/dx = y/x , y(1) = −2

Answer 1

Start with

$\dfrac{dy}{dx}=\dfrac{y}{x}$

Separate the variables:

$\dfrac{dy}{y} = \dfrac{dx}{x}$

Integrate both parts:

$\displaystyle \int \dfrac{dy}{y} = \int\dfrac{dx}{x}$

Which implies

$\log(y) = \log(x)+c$

Solving for y:

$y = e^{\log(x)+c} = e^{\log(x)}e^c=xe^c$

Since $e^c$ is itself a constant, let's rename it $c_1$.

Fix the additive constant imposing the condition:

$y(1) = c_1\cdot 1 = -2\iff c_1=-2$

So, the solution is

$y(x) = -2x$