It is customary to write this as [r,θ].
A point can be converted from polar to Cartesian coordinates using the formulas
\[ x = r \cos(\theta), \]
\[ y = r \sin(\theta). \]
Similarly, a point can be converted from Cartesian to polar coordinates using the formulas
\[ r = \sqrt{x^2 + y^2}, \]
\[ \theta = \arctan (y/x) . \]