也就是二重和三重积分的内容，以及平面和空间中的向量积分。

And so that was stuff about double and triple integrals and vector calculus in the plane and in space.

youdao

那么就是在这个区域的对xdA的二重积分，当然可能和密度有关系，但在这认为密度均为。

So, it's one over the area times the double integral of xdA, well, possibly with the density, 1 but here I'm thinking uniform density one.

youdao

在和简表演二重唱时，玛丽唱得非常好。

Mary sang very well when duetting with Jane.

youdao