Goi a la so to lop co the chia; 30;a,18;a,a∈Uc/30,18\,a nhieu nhat⇒a la UCLN/30,18\ 30bang2.3.5;18bang2.3mu 2 UCLNcua 30va18 la 2.3bang6;Vay lop chia thanh nhieu nhat la 6 to.Khi do to co so nam la 5 nam vi 30chia 6 to bang 5;;;;Khi do moi to co so nu la 3 nu vi 18chia 6to bang 3 nu