Which of the following differential equations are separable?
dydx=x+y\frac{dy}{dx} = x + ydxdy=x+y
dydx=xy\frac{dy}{dx} = xydxdy=xy
dydx=xy\frac{dy}{dx} = \frac{x}{y}dxdy=yx
dydx=x2+1\frac{dy}{dx} = x^2 + 1dxdy=x2+1