< CSP = < RSQ (vert. opp)
< SCP = < RAP (exterior of opp side of cyclic quad)
< RPA = < DPR (given)
.'. RPA ||| SCP
with < ARP = < CSP = <QSR = <QRS
< QRY = QSY (given)
so < QYR = < QYS = 90' (angle sum of triangle, QYR+QYS = 180)
.'. QY is perpendicular to RP