# Theorem of Green, theorem of Gauss and theorem of Stokes

## Theorem of Green

Let $$F(x,y)=(F_x(x,y),F_y(x,y))$$ be a differentiable function of two variables in the plane, and let $$D$$ be a region of the real plane. The border of $$D$$ is $$C$$.

Therefore:$$\displaystyle \int_C f\cdot dL=\int_D(\frac{d}{dx}F_y-\frac{d}{dy}F_x) \ dxdy$$$## Theorem of Gauss $$V$$ is a closed volume in space, and $$S$$ is its border parametrized (its "skin"), therefore, if $$F:V \subset \mathbb{R}^3 \longrightarrow \mathbb{R}^3$$ , it is a differentiable function in $$V$$, $$\displaystyle \int_S F \cdot dS=\int_V div(F)\cdot dxdydz$$$ With this theorem, we can convert complicated surface integrals into volume integrals.

### Procedure

1. Calculate $$div (F)$$
2. Find the integration region $$V$$ (a volume, so $$3$$ variables)
3. Calculate the integral with $$3$$ variables.

## Theorem of Stokes

A surface of space is $$S$$ and $$C$$ is its border (or limits), and let $$F:S \subset \mathbb{R}^3 \longrightarrow \mathbb{R}^3$$ be a differentiable function in $$S$$, then $$\displaystyle \int_C F \cdot dL=\int_S rot(F) \cdot dS$$\$

This theorem can be useful in solving problems of integration when the curve in which we have to integrate is complicated.

It also shows that if $$F$$ has rotational $$0$$ in $$S$$, then its integral along the curve $$C$$ is zero.

### Procedure

1. Find the parametrized integration region $$S$$ (a surface, so $$2$$ variables).
2. Calculate $$rot (F)$$.
3. Calculate the integral of $$2$$ variables of the rotacional of $$F$$.