ABCD is a quadrilateral whose sides AB, BC, CD and DA touches the circle at points P, Q, R and S, respectively. Remember: The lengths oftangents drawn from an external point to a circle are equal. ∴ AP = AS ....(1) PB = BQ ......(2) DR = DS ......(3) CR = CQ ......(4) Adding (1), (2), (3) and (4), we have (AP + PB) + (CR + DR) = (AS + DS) + (BQ + CQ) ⇒ AB + CD = AD + BC