X^2+2.5X+1≤0
（X+5/4）^2-25/16+1≤0
X+5/4）^2≤9/16
3/4≥X+5/4≥-3/4
-1/2≥X≥-2
则A={X|-1/2≥X≥-2}
4≥X^2≥1/4
4+a≥X^2+a≥1/4+a
若A交B为空集,则1/4+a>-1/2
a>-3/4