Question 17
$P$ is inside right angle xOy. $A$ lies on Ox, $B$ on Oy, and $APB = 90 ^{\circ}$. On the rays opposite $AP$ and $BP$, take $C , D$ with $CA = OA$ and $BD = OB$. $a$) Prove $C , O , D$ collinear. $b ) M$ is midpoint $CD$; prove $AMB = 90 ^{\circ}$.