Polar coordinates are such that and .
How do we write an integral in terms of and ?
Two options:
-
Partition a polar sector by concentric circles with radii and radial lines and then use the definition of the integral to find what changes to.
-
Linear algebra: Recall that the determinate tells us the area of a parallelogram given by the vectors that form its rows. Hence the change in area caused by changing coordinate systems can be found computing the Jacobian?
Theorem 1
If is some function in Cartesian coordinates, is some region in space, and and is the region written in coordinates then:
where is the area with the respect to and . We take the absolute values of the Jacobian matrix not just the determinate.
Theorem 2
In polar coordinates, .
Proof
In polar coordinates and . Hence,
\begin{align}
\frac{ \partial (x,y) }{ \partial (r,\theta) } & = \begin{vmatrix}
\frac{ \partial x }{ \partial r } & \frac{ \partial x }{ \partial \theta } \ \frac{ \partial y }{ \partial r } & \frac{ \partial x }{ \partial \theta } \end{vmatrix} \[3ex] & = \begin{vmatrix} \cos \theta & -r\sin \theta \ \sin \theta & r \cos \theta \end{vmatrix} \[3ex] &=r \end{align}
Hence $dA = | r |dr d\theta=r dr d\theta$. This means that\int \int _{R} f(x,y) , dA = \int \int _{s} f(r\cos(\theta),r\sin \theta)r , dr , d\theta