韦达定理
x1+x2=2
x1x2=1/2
(x1-2)(x2-2)=x1x2-2(x1+x2)+4=1/2-4+4=1/2
(x1-x2)^2=(x1+x2)^2-4x1x2=4-2=2
所以|x1-x2|=根号2