2x²-4x-1=2(x²-2x+1)-3=2(x-1)²-3
x²-2x-4=(x-1)²-5
因为,无论x取何值,（x-1）²≥0
所以总有:2(x-1)²-3＞(x-1)²-5
即：不论x取何值,代数式2x²-4x-1的值总大于x²-2x-4的值