2x²+5x-1-(x²+8x-4)
=2x²＋5x-1-x²-8x+4
=x²-3x+3
=(x-3/2)²＋3/4
≥3/4＞0
所以
无论
x取何值,代数式2x²+5x-1的值总比x²+8x-4大