ĐKXĐ : `x^2 + 1 \ge 0`
`⇔ x^2 \ge -1` (luôn đúng)
`⇒ x ∈ RR`
........................................
ĐKXĐ : `4x^2 + 3 \ge 0`
`⇔ 4x^2 \ge -3`
`⇔ x^2 \ge -3/4` (luôn đúng)
`⇒ x \in RR`
.........................................
ĐKXĐ : `9x^2 - 6x + 1 \ge 0`
`⇔ (3x)^2 - 2*3x*1 + 1^2 \ge 0`
`⇔ (3x-1)^2 \ge 0 ∀x`
`⇒ x ∈ RR`
...............................................
ĐKXĐ : `-x^2+2x-1 \ge 0`
`⇔ -(x^2-2x-1) \ge 0`
`⇔ -(x-1)^2 \ge 0`
`⇔ (x-1)^2 \le 0`
`⇔ x - 1 = 0`
`⇔ x = 1`
..................................................
ĐKXĐ : `-|x+5| \ge 0`
`⇔ |x+5| \le 0`
`⇔ |x+5| = 0`
`⇔ x + 5 = 0`
`⇔ x = -5`
................................................
ĐKXĐ : `-2x^2 -1 \ge 0`
`⇔ -2x^2 \ge 1`
`⇔ 2x^2 \le -1`
`⇔ x^2 \le -1/2` (vô lý)
`⇒ x ∈ ∅`
