ABCD is a parallelogram and AP and CQ are perpendiculars from vertices A and C on diagonal BD (See the given figure). Then,
AP=CQ
BP=DQ
ABCD is a parallelogram (given)
In△APD and △CQB
(i)AD=BC...(ABCD is a paralleligram)
(ii)∠APD=∠CQB=90∘ ...(given)
(iii) ∠ADP=∠CBQ ...(alternate angles)
(iv) △APD≅△CQB ...(AAS Postulate)
(v) AP=CQ ...(cpct)
(vi) DP=BQ ...(cpct)
Adding PQ to both sides we get,
DP+PQ=BQ+QP
DQ=BP