Given Let ABCD be a parallelogram and AP, BR, CR, be are the bisectors of ∠A, ∠B, ∠C and ∠D, respectively. To prove Quadrilateral PQRS is a rectangle. Proof Since, ABCD is a parallelogram, then DC || AB and DA is a transversal. ∠A+∠D= 180° [sum of cointerior angles of a parallelogram is 180°] ⇒ 1/2 ∠A+ 1/2 ∠D = 90° [dividing both sides by 2] ∠PAD + ∠PDA = 90° ∠APD = 90° [since,sum of all angles of a triangle is 180°] ∴ ∠SPQ = 90° [vertically opposite angles] ∠PQR = 90° ∠QRS = 90° and ∠PSR = 90° Thus, PQRS is a quadrilateral whose each angle is 90°. Hence, PQRS is a rectangle.