In Problems $15-20$ solve the given differential equation by using an appropriate substitution. Solve problem $16$ and $18.$

$15.x\frac{dy}{dx}y=\frac{1}{y^2}$

$16.\frac{dy}{dx}-y=e^x{y}^2$

$17.\frac{dy}{dx}=y(x{y}^3-1)$

$18.x\frac{dy}{dx}-(1x)y=x{y}^2$