Join QS. Let diagonals PR and QS intersect each other at T. We know, that diagonals of a parallelogram bisect each other . ∴ T is the mid - point of QS. Since a median of a triangle divides it into two triangles of equal area. ∴ In △PQS , PT is its median . ⇒ ar(△PTS) = ar(△PQT) ---i) In △SQO, OT is its median . ⇒ ar(△STO) = (△QTO) ---ii) Adding (i) and (ii) , we have ar(△PTS) + ar( △STO) = ar(△PQT) = ar( △ QTO ) ⇒ ar(△PSO) = ar(△PQO)