Hi Everyone.

I am currently reading a book called "An Introduction to the Finite Element Method" by Reddy (Third Edition).

In the book he has a chapter on mathematical preliminaries. I am completely stuck on something he wrote entitled "Gradient Theorem" which is completely different from what I learned from Multivariable Calculus. He says the following

The integral of (The gradient of the scalar function F) dxdy (closed surface) is equal to the line integral around the boundary of (nF) ds

Where did this come from and may anyone provide the link to online readable text on this?

Any help would be greatly appreciated. Thanks.