These coordinate system are two equivalent ways of representing a vector. Use the coordinate system that is most appropriate for the problem.

Cartesian coordinates:   A  =  (A_x,A_y) = A_x + A_y

Polar coordinates:   A = A_r  if the vector begins at the origin. Sometimes we write such a vector as A =(r,θ) Here θ is the angle between the positive x-axis and A. More generally, A = (A_r,A_θ); (see transparency.

Conversion between cartesian and polar coordinates: see transparency.