Cho n diem ( n ≥2). Noi tung cap 2 diem trong n diem do thanh cac doan thang
a) hoi co bn doan thang ne trong n diem do ko co ba diem nao thang hang
b)hoi co bn doan thang neu trong n diem do co dung 3 diem thang hang
c)tinh n biet co tat ca 1770 doan thang