Remember the general method is that we first look at the inner integral, here integral of f dy.

一般是,先考虑内积分，这里即f对dy的积分。

youdao

Namely, let's say that you have a function maybe of three variables, x, y, z, df = fxdx+fydy +fzdz then you would write df equals f sub x dx plus f sub y dy plus f sub z dz.

即是说，如果你有一个,含自变量，x，y，z的函数，那么。

youdao