The entire second term with dy goes away because dy is zero. And, in the first term, 0 8xy goes away because y is zero as well.

这个带dy的第二项消去,因为dy是0，第一项中，8xy消去，因为y也是。

youdao

So, if we want to set it up instead dx dy, then it means we are going to actually look at the converse question which is, for a given value of y, what is the range of values of x?

如果要用dxdy来替换，就是说,转化为与之相反的问题来看，给定y的一个值，那么x的取值范围是什么呢？

youdao

the general notation for vector dr-- y equals dx, x roof, plus dy, y roof, z plus dz, z roof.

总的向量dr的符号-,=dr向量x+dy向量，+dz向量。

youdao