设二个交点坐标是(x1,0),(x2,0)
x1+x2=-a
x1x2=a-2
|x1-x2|^2=(x1+x2)^2-4x1x2
(2根号5)^2=a^2-4(a-2)
a^2-4a-12=0
(a-6)(a+2)=0
a=6或-2|x1-x2|^2=(x1+x2)^2-4x1x2....后面的为何-4x1x2这是公式展开的啊,(x1-x2)^2=x1^2-2x1x2+x2^2=(x1^2+x2^2+2x1x2)-4x1x2=(x1+x2)^2-4x1x2.