Given AC and PQ intersect each other at the point O and AB II DC.
To prove, OA.CQ = OC. AP
Proof in ΔAOP and ΔCOQ, ∠AOP=∠COQ [vertically opposite angles]
[since, AB II DC and PQ is transversal, so alternate angles]
∴ ΔAOP∼ΔCOQ [by AAA similarity criterion]
Then, OAOC=APCQ [since, corresponding sides are propotion]
⇒ OA.CQ = OC.AP