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。
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？
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。
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。
OK, so I'm computing the integral of y dx x dy.
我来计算ydx+xdy的积分。
OK, so I'm computing the integral of y dx x dy.
我来计算ydx+xdy的积分。
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