PQRS is a parallelogram. L and M are points on PQ and SR respecitvely such that PL = MR. Show that LM and QS bisect each other.
Let LM and BAD intersect at O
as AL = CM and AB = CD
therefore AB-AL=CD-CM
BL = DM ___ (i)
consider ΔOMD and ΔOLB
∠DOM=∠BOL (vertically opposite angles)
∠OMD=∠OLB (alternate interior angles)
DM = BL (from i)
therefore Δ OMD congruent to ΔOLB
therefore OM = OL,
OD = OB (cpctc)
Therefore O bisects LM and BD
therefore LM, BD bisect each other
(here parellelogram is ABCD)