Which substitution transforms y′=yx+tan(yx)y' = \frac{y}{x} + \tan(\frac{y}{x})y′=xy+tan(xy) into a separable ODE?
u=xyu = xyu=xy
u=x/yu = x/yu=x/y
u=y/xu = y/xu=y/x
u=y+xu = y+xu=y+x