Dz/Dx = f1*y + f2*(2x) = y*f1 + 2x*f2,
D²z/Dx² = (D/Dx)(y*f1 + 2x*f2) = [y*(y*f11 + 2x*f12)+ 2*f2 + 2x*(y*f21 +2x*f22)] = ……,
D²z/DxDy = (D/Dy)(y*f1 +2x*f2) = [f1 + y*(x*f11 + 2y*f12)+ 2x*(x*f21 +2y*f22)] = …….
Dz/Dx = f1*y + f2*(2x) = y*f1 + 2x*f2,
D²z/Dx² = (D/Dx)(y*f1 + 2x*f2) = [y*(y*f11 + 2x*f12)+ 2*f2 + 2x*(y*f21 +2x*f22)] = ……,
D²z/DxDy = (D/Dy)(y*f1 +2x*f2) = [f1 + y*(x*f11 + 2y*f12)+ 2x*(x*f21 +2y*f22)] = …….