4x^2-4(a-1)x+a^2-2a-8=0
4x^2-4(a-1)x+(a-1)^2-9=0
{2x-(a-1)}^2=9
2x-(a-1)=正负3
2x=a-1+3 或 2x=a-1-3
x1=a/2+1 x2=a/2-2