Which substitution turns y′=x+yx−yy' = \frac{x+y}{x-y}y′=x−yx+y into a separable equation?
y=vxy = vxy=vx
v=x+yv = x+yv=x+y
v=x−yv = x-yv=x−y
y=v2y = v^2y=v2