Euclidean geometry, which means you are allowed to use the parallel postulate and its consequences already established
16. Let y be a circle with center O and let P, Q, and R be three points
on y. Prove that if P and R are diametrically opposite, then PQR is a right angle, and if O and Q are on the same side of PR, then (PQR) = 1/(XPOR). (Hint: Again use the fact that the triangu- lar angle sum is 180. There are four cases to consider, as in Fig ure 5.18.) State and prove the analogous result when O and Q are on opposite sides of PR.
P
R
Q
R
0
O
P
R
0
0
0
R