` 2a \vdots a - 1`
` => 2*(a-1) +2 \vdots a -1`
Ta có ` a -1 \vdots a -1 => 2(a-1) \vdots a -1`
` => 2 \vdots a -1`
` => a -1 ∈ Ư(2) = { -2; - 1; 1 ; 2 }`
` a -1 = -2 => a =-1`
` a -1 = -1 => a = 0`
` a -1 = 1 => a =1`
` a - 1 = 2 => a =3`
Vậy ` a ∈ { -1 ; 0 ; 1 ; 3}`
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` 3a- 8 \vdots a -4`
` => 3*(a-4) + 4 \vdots a -4`
` => 4 \vdots a -4`
` => a -4 ∈ Ư(4) = { -4 ; -2 ; 2 ;4 }`
` a -4 = -4 => a = 0`
` a -4 = -2 => a = 2`
` a -4 = 2 => a= 6`
` a - 4 = 4 => a =8`
Vậy ` a ∈ { 0 ; 2 ; 6 ; 8 }`
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` a^2 +a +1 \vdots a +1`
` => a(a+1) + 1 \vdots a +1`
Ta có `a +1 \vdots a +1 => a(a+1) \vdots a +1`
` => 1 \vdots a +1`
` => a +1 ∈ Ư(1) = { -1; 1}`
` a +1 = -1 => a= -2`
` a +1 = 1 => a =0`
Vậy ` a ∈ {-2;0}`
