And, just to motivate that, let me remind you about one trick that you probably know from single variable calculus, namely implicit differentiation.

作为引子，你们可能已经知道了，一元微积分里面的一个小把戏，也就是求隐函数微分法。

You can get this either by implicit differentiation and the product rule, or you could get this just by putting here, here,and here the partial derivatives of this with respect to x, y, and z.

可以通过隐函数微分法和乘法法则得到，或者直接,把关于x，y，z的偏微分放进来。