a/ \(-2x^2+8x-18\\=-2x^2+8x-8-10\\=-(2x^2-8x+8)-10\\=-2(x^2-4x+4)-10\\=-2(x-2)^2-10\)
Vì \(-2(x-2)^2\le 0→-2(x-2)^2-10\le -10∀x\)
b/ \(2x^2-4x+18\\=2x^2-4x+2+16\\=2(x^2-2x+1)+16\\=2(x-1)^2+16\)
Vì \( 2(x-1)^2\ge 0→2(x-1)^2+16 \ge 16∀x\)
c/ \(-x^2+4x\\=-x^2+4x-4+4\\=-(x^2-4x+4)+4\\=-(x-2)^2+4\)
Vì \(-(x-2)^2\le 0→-(x-2)^2+4\le 4\)