在1-20的自然數(shù)中,奇數(shù)有
1,3,5,7,9,11,13,15,17,19
1,3,5,7,9,11,13,15,17,19
,偶數(shù)有2,4,6,8,10,12,14,16,18,20
2,4,6,8,10,12,14,16,18,20
質數(shù)有2,3,5,7,11,13,17,19
2,3,5,7,11,13,17,19
,合數(shù)有4,6,8,9,10,12,14,15,16,18,20
4,6,8,9,10,12,14,15,16,18,20
.