a-20-->a2
delta=a^2-8(a-2)=a^2-8a+16=(a-4)^2>=0
|x1-x2|(x1-x2)^2=(x1+x2)^2-4x1x2=a^2/(a-2)^2-8/(a-2)=0
因此得取值范围是：a>2.5 or a漏了一个等于