F(x+y,x+z)=1F1+F2(∂z/∂y)=0 ∂z/∂y= -F1/F2∂²z/∂y²=[-F2(F11+F12(∂z/∂y))+F1(F21+F22(∂z/∂y)]/F2^2=[-F2(F11+F12(-F1/F2))+F1(F21+F22(-F1/F2)]/F2^2=[-F2(F2F11+F12(-F1))+F1(F2F21+F22(-F1)]/F2^3=[-F2^2F11+2F1F2F12-F1^2F22]/F2^3