Given ∠TPQ=∠RQP=90∘
Sum of angles, ∠TPQ+∠RQP=90+90=180∘
Since, the sum of angles is 180, hence, PT∥RQ
In △PST and △PQR,
∠PST=∠PQR (each 90∘)
∠TPS=∠PRQ (Alternate angles for parallel lines PT and RQ)
∠STP=∠RPQ (Third angle)
Thus, △PST∼△RQP
Hence, PSQR=STPQ
PS×PQ=ST×QR