Bài 4:
`1, A = -x^2 - 2x -2`
`= - ( x^2 + 2x +1 +1 )`
`= - ( x+ 1 )^2 -1` $\leqslant$ `-1 < 0`
Vậy `A` luôn âm với `∀x`
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`2, B = - x^2- 4x -7`
`= - ( x^2 + 4x + 4 + 3)`
`= - ( x + 2 )^2 - 3` $\leqslant$ `-3 < 0`
Vậy `B` luôn âm với `∀x`
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`3, C = -x^2 -6x -11`
`= - ( x² + 6x +9 +2 )`
`= - ( x + 3 )^2 -2` $\leqslant$ `-2 < 0`
Vậy `C` luôn âm với `∀x`
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`4, D = -x^2 -x -1`
`= - ( x^2 + x +1 )` $\\$ `= - ( x + 1/2 )^2 - 3/4` $\leqslant$ `-3/4 < 0`
Vậy` D` luôn âm với `∀x`
`#MizuCutee#`
