[SOLVED] A point that divides a line segment in a given ratio in polar

Say we are given the Cartesian points P1=(x1, y1) and P2=(x2, y2).

P1-------------------P3-----------------------------------P2

|----------m--------| |----------------n-------------------|

We want to find the coordinates of P3. Use this formula:

$\displaystyle ([n*x_1 + m*x_2]/[m+n] , ([n*y_1 + m*y_2]/[m+n])$

Now what if the points were in polar: P1=(r1, theta1) and P2=(r2, theta2). What is the equivalent equation for polar coordinates?