用配方法证明:代数式2x-x2-5的值恒小于零
2x-x²-5
=-x²+2x-5
=-(x²-2x+1)-4
=-(x-1)²-4
∵ 平方非负,
∴ -(x-1)²≤0
∴ y=-(x-1)²-4