ABCD is a parallelogram in which P, Q, R and S are the mid points of sides AB, BC CD and DA respectively. Then,
PQ = RS
In ∆ABC, P and Q are the mid points of sides AB and BC respectively.
By mid point theorem, PQ || AC and PQ = ½ AC
In ∆ADC, R and S are the mid points of sides CD and AD respectively.
RS || AC and RS = ½ AC
Thus you get PQ = RS