The plane polar coordinates (r, u ) identify the position of a point P in the plane by giving its radial distance r from a fixed origin O (the pole), and the
angle u measured anticlockwise in radians from a fixed reference line
through O called the polar axis, to the radial line joining the origin to point
P. The polar coordinate system is shown in Fig. 20. This figure shows also
that the polar angle u is not unique because for any given r and u the point
P (r, u) will be the same as the points P (r, u9/2np ) for n/0, 1, 2, . . . . To remove this ambiguity, it is usual to confine u to the interval 00/u0/2p.