公因倍數(數學)

1) 81,108的h.c.f
81 = 3 x 3 x 3 x 3
108 = 2 x 2 x 3 x 3 x 3
HCF = 3 x 3 x 3 = 27
2)12和30的第7個公倍數
= (12和30的LCM) x 7
12 =2 x 2 x 3
30 = 2 x 3 x 5
LCM = 2 x 2 x 3 x 5 = 60
第7個公倍數 = 60 x 7 = 420