>>194
∂F/∂x = (∂f/∂x)(∂g/∂x) + (∂f/∂y)(∂h/∂x) = a (∂f/∂x) + c (∂f/∂y)
∂F/∂y = (∂f/∂x)(∂g/∂y) + (∂f/∂y)(∂h/∂y) = b (∂f/∂x) + d (∂f/∂y)

∂(g,h)/∂(x,y) = ad-bc