Solution
= Solution
In <plane polar coordinates>, $r$ is distance from the origin and $\theta$ is the angle from positive $x$ toward positive $y$. The conversion to <Cartesian coordinates> is
$$
\boxed{x=r\cos\theta,\qquad y=r\sin\theta}.
$$
Conversely $r=\sqrt{x^2+y^2}$, with the angular coordinate undefined at the origin.