Cho hinh binh hanh abcd goi o la giao diem cua hai duong cheo ac va bd tren ab lay diem e tren cd lay diem f sao cho ae=cf 
cm rang tam giac aeo=tam giac cfo
cm rang e va f doi xung voi nhau qua o 
qua e ke duong thang song song coi ac cat bc tai i qua f ke duong thang song song voi ac cat ad tai k .cm rang tu giac keif la hbh