Since OQ = OR, angle(OQR) = angle(ORQ)
angle(O) + angle(OQR) + angle(ORQ) = 180.
angle(O) = 180 - angle(OQR) - angle(ORQ)
angle(O) = 180 - 2·angle(ORQ)
angle(O) = arc(QR)
angle(QPR) = ½·arc(QR) = ¼·angle(O) ---> 2·angle(QPR) = angle(O)
---> 2·angle(QPR) = 180 - 2·angle(ORQ)
2·angle(QPR) + 2·angle(ORQ) = 180
angle(QPR) + angle(ORQ) = 90