Question 21
In the diagram at the right, $AB$ is the diameter of the circle with centre $O$. Two circles are drawn with $AO$ and $OB$ as diameters. In the region between the circumferences, a circle with centre $D$ is inscribed to touch the other three circles. If the length of the radius of the circle $D$ is 8 cm, find the length, in cm, of $AB$.
