Applying triangle law of addition of vector In APAC and PSC, we get PA + AC = PC and PB + BC = PC On adding both equations, PA + AC + PB + BC = PC + PC ⇒ PA + PB + AC + BC = 2PC ⇒ PA + PB + AC - AC = 2PC  (∵ C is the mid-point of AB) =* PA+ PB = 2PC