In figure PA and PB are tangents from P to the circle with centre O. R is a point on the circle. Prove that : PC + CR = PD + DR
Since the tangents from an external point to a circle are equal in length, therefore
PA = PB, CA = CR and DB = DR
Now, PA = PB
⇒ PC + CA = PD + DB
⇒ PC + CR = PD + DR.