∂w/∂x=f1'∂(x+y+z)/∂x+f2'∂(xyz)/∂x
=f1'*1+f2'*yz
(∂²w)/(∂x∂z)
=∂(∂w/∂x)/∂z
=∂(f1'*1+f2'*yz)/∂z
=f11''*∂(x+y+z)/∂z+f12''∂(xyz)/∂z+f12''*(yz)*∂(x+y+z)/∂z+f22''*(yz)*∂(xyz)∂z+f2'*∂(yz)/∂z
=f11''+xyf12''+yzf12''+xy^2zf22''+yf2'
=f11''+(xy+yz)f12''+xy^2zf22''+yf2'