KöMaL 2015 B4752

Consider circles $k_1$ with center $O_1$ and radius $r_1$, and $k_2$ with center $O_2$ and radius $r_2$. Line $PA$ is tangent to $k_1$ at $A$ and line $PD$ is tangent to $k_2$ at $D$ . Segment $AD$ intersects $k_1$ and $k_2$ at $B$ and $C$ , respectively. Find $PA/PD$ in terms of $r_1$ and $r_2$ if $AB=CD$ .