Bài 2:
`a) 15 vdots n-1`
`=>n-1 in Ư(15)={+-1; +-3; +-5; +-15}`
`=>n in {0; 2; -2; 4; 6; -4; -14; 16}`
Vì `n in N=>n in {0;2;4;6;16}`
`b) 12 vdots n-3`
`=>n-3 in Ư(12)={+-1;+-2;+-3;+-4;+-6;+-12}`
`=>n in {4; 2; 1; 5; 6; 0; 7; -1; -3; 9; -9; 15}`
Vì `n in N=>n in { 4; 2; 1; 5; 6; 0; 7; 9; 15}`
Bài 3
Vì `10:3` `dư` `1=>10^20:3` `dư` `1`
Vì `8:3` `dư` `2`
`=>10^20+ 8 vdots 3 (đpcm)`
Vì `10 vdots 2; 8 vdots 2`
`=>10^20+ 8 vdots 2`
Mà `10^20+ 8 vdots 3`
`=>10^20+ 8 vdots (2.3)`
`10^20+ 8 vdots 6 (đpcm)`
Bài 4:
Vì `15 vdots 3; 9 vdots 3`
`=>b vdots 3`
