`a.`

`A = {` `2; 6; 12; 20; 30` `}`

Ta có :

`2 = 1.2`

`6 = 2.3`

`12 = 3.4`

`20 = 4.5`

`30 = 5.6`

`→` `A = {` `x(x+1) | x ∈ NN^** ; 1 ≤ x ≤ 5` `}`

`b.`

`A = {` `2; 3; 5; 7` `}`

`→` `A = {` `x | x` là số nguyên tố `;` `2 ≤ x ≤ 7` `}`

`c.`

`A = {1/2; 1/6; 1/12; 1/20; 1/30}`

Ta có :

`1/2 = 1/(1.2)`

`1/6 = 1/(2.3)`

`1/12 = 1/(3.4)`

`1/20 = 1/(4.5)`

`1/30 = 1/(5.6)`

`→` `A = { 1/(x(x+1)) | x ∈ NN^** ; 1 ≤ x ≤ 5 }`