Đáp án:
`a)`
` x^2 -4x +9 = x^2 -4x + 4 + 5 = (x^2-4x+4) +5 = (x-2)^2 +5`
Ta có : `(x-2)^2 \ge 0 => (x-2)^2 +5 \ge 5 => (x-2)^2 +5 > 0 (dpcm)`
`b)`
` 1 - x +x^2 = x^2- x+ 1 = (x^2 -x +1/4)+ 3/4 = (x-1/2)^2 +3/4`
Ta có ` (x-1/2)^2 \ge 0 => (x-1/2)^2 +3/4 \ge 3/4 => (x-1/2)^2 +3/4 > 0 (dpcm)`
`c)`
` 10 -6x + x^2 = x^2- 6x +10 = (x^2-6x+9) +1 = (x-3)^2 +1`
Ta có ` (x-3)^2 \ge 0 => (x-3)^2+1 \ge 1 > 0 (dpcm)`
`d)`
` 4x^2 + 4x +2007 = (4x^2 +4x +1) + 2006 = (2x+1)^2 +2006`
Ta có `(2x+1)^2 \ge 0 => (2x+1)^2 +2006 \ge 2006 > 0(dpcm)`
