Cartesian coordinates such as discussed in Section 1.1 are not the only way to represent points in the plane. We can also represent them by indicating the distance, \(r\text{,}\) from the origin and the angle, \(\varphi\text{,}\) from the \(x\)-axis as shown in Figure 5.22.
Figure5.22.Polar coordinates of a point in the plane
If we assume that \(r>0\) and \(\varphi\in[0,2\pi)\) then every point except the origin has the unique representation
\begin{align*}
x \amp =r\cos\varphi\\
y \amp =r\sin\varphi\text{.}
\end{align*}
Hence \(\mathbb R^2\) is the image of the strip \([0,\infty)\times[0,2\pi)\) under the transformation
We want to apply the transformation formula from the previous section to find out how to integrate a function given in polar coordinates. To do so we need to compute the Jacobian determinant of \(\vect g\text{.}\) The Jacobian matrix is given by