dydx=f(x)g(y)\dfrac{dy}{dx} = f(x) g(y)dxdy=f(x)g(y) ⇒\Rightarrow⇒ ∫dyg(y)=∫f(x)dx\int \dfrac{dy}{g(y)} = \int f(x) dx∫g(y)dy=∫f(x)dx.