$\mathrm{O}$ and $\mathrm{O}\text{'}$ are the centers of two circles that touch each other externally at $\mathrm{P}$. $\mathrm{AB}$ is a common tangent. Find the value of $\angle APB$.

In the given figure, $\mathrm{AB}$ and $\mathrm{CD}$ are two common tangents to the two touching circle. If $\mathrm{CD}=6 \mathrm{cm}$, then $\mathrm{AB}$ is equal to :

$\mathrm{O}$ and $\mathrm{O}\text{'}$ are the centers of circle of radii $20 \mathrm{cm}$ and $37 \mathrm{cm}$. $\mathrm{AB}=24 \mathrm{cm}$. What is the distance $\mathrm{OO}\text{'}$?