x1,x2为方程x^2+3x+1=0的两实数根
x1+x2=-3
x1^2+3x1=-1
x1^2-3x2+20
=x1^2+3x1-3x1-3x2+20
=x1^2+3x1-3(x1+x2)+20
=-1+9+20
=28