DM=LB (both are half of equal sides AB and CD)
and
DM paralle to LB (as AB and CD are parallel)
So, DLBM is a parallelogram.(one pair of opposite sides is parallel and equal)
So we get, DL parallel to MB.
Suppose DL and BM intersect AC in P and Q respectively.
Consider triangle ABC,
PL parallel to QB ( as DL parallel to MB)
L is the midpoint of AB,
so P must be the mid point of AQ( conv of mid pt thm)
So,
AP=PQ..(i)
Simly, considering triangle CDP,
we get
CQ=QP..(ii)
from (i) and (ii), we get,
AP=PQ=CQ.
Thus,
P and Q trisect the diagonal AC.
Like if satisfied