Ta có:
AB song song CD <=> AB song song CK
=> Goc ABE = goc ECK so le trong
Xet hai tam giac ABE va tam giac KCE ta co:
+) Goc ABE = goc ECK
+) Canh BE = canh EC ( E la trung diem cua BC)
+) Goc AEB = goc CEK ( doi dinh)
=> Tam giac ABE = tam giac KCE (gcg)