`a)`
- Ta có : `x+13 vdots x-5`
mà `x-5 vdots x-5`
`=> 18 vdots x-5`
`=> x-5 in Ư(18)={1;2;3;6;9;18}`
`=> x in {6;7;8;11;14;23}`
`b)`
- Ta có : `3x+1 vdots 11-2x` và `11-2x vdots 11-2x`
`=> 3x+1+(11-2x) vdots 11-2x`
`=> 2(3x+1)+3(11-2x) vdots 11-2x`
`=> 6x+2+(33-6x) vdots 11-2x`
`=> (6x-6x)+(2+33) vdots 11-2x`
`=> 35 vdots 11-2x`
`=> 11-2x in Ư(35)={1;5;7;35}`
`=> 2x in {10;6;4}`
`=> x in {5;3;2}`
`c)`
- Ta có : `3x+9 vdots 2x-3` và `2x-3 vdots 2x-3`
`=> 3x+9 - (2x-3) vdots 2x-3`
`=> 2(3x+9)-3(2x-3) vdots 2x-3`
`=> 6x+18-(6x-9) vdots 2x-3`
`=> 6x+18-6x+9 vdots 2x-3`
`=> (6x-6x)+(18+9) vdots 2x-3`
`=> 27 vdots 2x-3`
`=> 2x-3 in Ư(27)={1;3;9;27}`
`=> 2x in {4;6;12;30}`
`=> x in {2;3;6;15}`
`d)`
- Ta có : `27-3x vdots 2x+3` và `2x+3 vdots 2x+3`
`=> (27-3x)-(2x+3) vdots 2x+3`
`=> 2(27-3x)-3(2x+3) vdots 2x+3`
`=> (54-6x)-(6x+9) vdots 2x+3`
`=> 54-6x-6x-9 vdots 2x+3`
`=> (54-9)-(6x-6x) vdots 2x+3`
`=> 45 vdots 2x+3`
`=> 2x+3 in Ư(45)={1;3;5;9;15;45}`
`=> 2x in {0;2;6;12;42}`
`=> x in {0;1;3;6;21}`
mà `x<=9`
`=> x in {0;1;3;6}`
