Chứng minh AD = BC

Cho tam giac ABC, M la trung diem cua canh AC. Tren tia doi cua tia MB lay diem D sao cho MD = MB.
a, Chung minh AD =BC.
b, Chung minh AB //CD.
c, Goi N la trung diem cua canh AB, tren tia doi cua tia NC lay diem E sao cho NC=NE.Chung minh rang A la trung diem cua DE.
Mong moi nguoi giai giup minh nha thank you!