x1+x2=-3
x1x2=1
x1²＋3x1+1=0
x1²=-3x1-1
所以
x1ˆ3+8x2+20
=x1 (x1²)+8x2+ 20
=x1·(-3x1-1)+8x2+ 20
=-3x1²-x1+8x2+ 20
=-3(-3x1-1)-x1+8x2+ 20
=9x1+3-x1+8x2+ 20
=8x1+8x2+23
=8(x1+x2)+23
=8×(-3)+23
=-24+23
=-1