X1,X2为方程x的平方+3X+1=0的两实根,
x1²+3x1+1=0
x2²+3x2+1=0
x1x2=1,x1+x2=-3
所以
X1的平方—3乘X2+20
=-3x1-1-3x2+20
=-3(x1+x2)+19
=-3×（-3）+19
=28