Since the quadrilateral ABCD formed by joining the mid – points of quadrilateral PQRS is a rectangle.
∴ AC = BD [since, diagonals of a rectangle are equal]
⇒ PQ = QR
Thus, quadrilateral PQRS is a rhombus.
Hence, diagonals of PQRS i.e., PR and QS are perpendiculars.
[since diagonals of rhombus are perpendicular to each other]