Answers to this question Homogeneous differential equation dydx=yx solution? assert that to find a solution to the differential equation dydx=yx we may rearrange and integrate ∫1y dy=∫1x dx. If we perform the integration we get logy=logx+c or y=kx for constants c,k∈R. I've seen others use methods like this before too, but I'm unsure why it works.
Question: Why is it legitimate to solve the differential equation in this way?
Answer
You start with
y′=yx⟹y′y=1x⟹∫y′dxy=∫dxx,
and you make the change of variables in the first integral, which results in what you've written
∫dyy=∫dxx
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