2x2+x+3
=2(x²+x/2)+3
=2(x+1/4)²+3-1/8
=2(x+1/4)²+23/8
因（x+1/4)²≥0
所以2(x+1/4)²+23/8≥23/8＞0
即代数式2x2+x+3的值恒大于零