1, cho tu giac ABCD co AB = BC = DA va goc A + goc C = 180 do . Chung minh rang :
a, BD la tia phan giac cua goc D
b, ABCD la hinh thang can
2, Cho tam giac ABC can tai A . Ke duong phan giac BMCN
a, tu giac BCMN la hinh gi ? Vi sao ?
b, Chung minh : BM = MN = CM