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。

youdao

If when x is low, y also tends to be low, then this will be negative number and so will this, so their product is positive.

如果x取值小,同时y值也小,这将是一个负值,这个也是负值,负负得正,结果是正值

耶鲁公开课 - 金融市场课程节选

Ok -- before I get too much email suggesting that X product can do this portion and Y product can do another portion, I'm not saying that converting is impossible.

ENGADGET: The Clicker: Dear Microsoft, "help me help you."