Polar Coordinates
1. Video: https://www.khanacademy.org/math/precalculus/parametric_equations/polar_coor/v/polar-coordinates-1
2. Explanation & 10 practice problems: http://www.mathsisfun.com/polar-cartesian-coordinates.html
2. Explanation & 10 practice problems: http://www.mathsisfun.com/polar-cartesian-coordinates.html