>>696
f(x,y)で y = g(x) のとき、xでの微分の計算

f(x+h, g(x+h)) - f(x,g(x))
= {f(x+h, g(x+h)) - f(x,g(x+h))} + {f(x, g(x+h)) - f(x,g(x))}

より
(d/dx) f(x, g(x)) = {(∂/∂x) f(x,y) } + {(∂/∂y) f(x,y) } (d/dx) g(x)