Since, tangents drawn from an exterior point to a circle are equal in length.
∴ AP = AS    ...(i)BP = BQ    ...(ii)CR = CQ    ...(iii)and DR = DS    ...(iv)Adding (i), (ii), (iii) and (iv), we getAP + BP + CR + DR = AS + BQ + CQ + DS⇒ (AP + BP) + (CR + DR)= (AS + DS) + (BQ + CQ)⇒ AB + CD = AD + BCHence, AB + CD = BC + DA.