✔ 最佳答案
ABCD is parallelogram
angleDAM=angleBCM (alt angles,AB//CD)
angleMDA=angleMBC (alt angles,AB//CD)
triangleMDA congruent to triangle MBC
DM=MB (corresponding sides, congruent triangles)
AM=MC (corresponding sides, congruent triangles)
AC和BD互相平分