chia 9 chiec nhan thanh 3 nhom, moi nhom 3 cai. Lan thu nhat de 2 nhom bat ki len dia can. Se co 2 TH:
TH1: can thang bang:
Lay nhom con lai chia thanh 3 phan, moi phan 1 cai. Lay 2 phan bat ki dat len dia can, neu can thang bang thi chiac nhan con lai la chiec nhe hon, neu can lech ve 1 phia thi phan nhe hon la chiec nhan bi lech la chiec nhan bi lech.
TH2: can bi lech:
Lay phan nhe hon chia thanh 3 phan, moi phan 1 cai. Lay 2 phan bat ki dat len dia, neu can thang bang thi chiac nhan con lai la chiec nhe hon, neu can lech ve 1 phia thi phan nhe hon la chiec nhan bi lech la chiec nhan bi lech.