`[2n+4]/[2n+1]=1+3/[2n+1]`
Để `2n+4 \vdots 2n+1` thì `1+3/[2n+1] in ZZ`
`=>3/[2n+1] in ZZ`
`=>2n+1 in Ư_3`
Mà `Ư_3={+-1;+-3}`
`@2n+1=-1=>2n=-2=>n=-1` (ko t/m)
`@2n+1=1=>2n=0=>n=0` (t/m)
`@2n+1=-3=>2n=-4=>n=-2` (ko t/m)
`@2n+1=3=>2n=2=>n=1` (t/m)
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`[6n+18]/[2n+1]=[6n+3+15]/[2n+1]=3+15/[2n+1]`
Để `6n+18 \vdots 2n+1` thì `3+15/[2n+1] in ZZ`
Hay `15/[2n+1] in ZZ`
`=>2n+1 in Ư_{15}`
Mà `Ư_[15]={+-1;+-3;+-5;+-15}`
`@2n+1=-1=>2n=-2=>n=-1` (ko t/m)
`@2n+1=1=>2n=0=>n=0` (t/m)
`@2n+1=-3=>2n=-4=>n=-2` (ko t/m)
`@2n+1=3=>2n=2=>n=1` (t/m)
`@2n+1=-5=>2n=-6=>n=-3` (ko t/m)
`@2n+1=5=>2n=4=>n=2` (t/m)
`@2n+1=-15=>2n=-16=>n=-8 `(ko t/m)
`@2n+1=15=>2n=14=>n=7` (t/m)