Consider the equation (y+x2+y2)dx−xdy=0(y + \sqrt{x^2+y^2})dx - x dy = 0(y+x2+y2)dx−xdy=0. What substitution makes this separable?
y=vxy = vxy=vx
x=vyx = vyx=vy
y=v+xy = v+xy=v+x
y=v2xy = v^2xy=v2x