x1,x2是方程的两根
则x1+x2=5/2,x1*x2=1/2
(x1-1)^2+(x2-1)^2
=x1^2+x2^2-2(x1+x2)+2
=(x1+x2)^2-2x1*x2-2(x1+x2)+2
=(5/2)^2-2*(1/2)-2*(5/2)+2
=9/4