[SOLVED] Equation for a point on a line in polar coords

[FONT="]Suppose we are given the Polar points P1=(r1, θ1) and P2=(r2, θ2) and we are given a distance D1. How would I find the coordinates of the point P3=(r3, θ3) that lies along the line segment between P1 and P2, and is a distance D1 from point P1?

It would look something like this:

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

|------D1-----|

So we know P1, P2 and distance D1.

I need a formula to find the point P3.

I would like to be able to solve this without converting to Cartesian coordinates because it is in a program that needs to loop as fast as possible - I've done it with the conversion and it is just taking too long. Not to mention the minor precision loss.

Any thoughts?

Please grace me with your vast knowledge, O' great Internets.