`1. A = {x ∈ RR, x² - 7x + 10 = 0}`
`=> A = {2; 5}`
`2. B = {x ∈ NN, x³ - 3x² + 7x - 5 = 0}`
`=> B = {1}`
`3. C = {x ∈ NN, x^4 - 4x² + 3 = 0}`
Đặt `x² = t (t ≥ 0)`
`=> t² - 4t + 3 = 0`
`<=>` \(\left\{ \begin{array}{l} t = 3\\t = 1\end{array} \right.\)
Với `t = 1 => x = ±1`
Với `t = 3 => x = ±sqrt{3}`
Mà: `x ∈ NN`
`=> C = {±1}`
`4,`
Với `x² ≥ 2`
`<=> |x| ≥ sqrt{2}`
`=>` \(\left[ \begin{array}{l}x ≥ \sqrt{2}\\x ≤ -\sqrt{2}\end{array} \right.\)
Mà: `x ≤ 6`
`=>` \(\left[ \begin{array}{l}\sqrt{2} ≤ x ≤ 6\\x ≤ -\sqrt{2}\end{array} \right.\)
`=> D = {2; 3; 4; 5; 6}`
`5`
`0 ≤ k ≤ 4, k ∈ NN`
`=> k ∈ {0; 1; 2; 3; 4}`
`=> x = 2k² + 1 ∈ {1; 3; 9; 19; 33}`