19. The diagram shows a segment of a circle such that $C D$ is perpendicular bisector of the chord $A B$. Given that $A B=16$ and $C D=4$, find the diameter of the circle.
19 Ans: 20 .
Let $r$ be the radius. Then $r^{2}=(r-4)^{2}+8^{2} \Longrightarrow r=10$. Thus the diameter is 20 .