x1²+3x1+1=0
x1²=-3x1-1
所以x1³
=x1(-3x1-1)
=-3x1²-x1
=-3(-3x1-1)-x1
=8x1+3
且x1+x2=-3
所以原式=8x1+3+8x2+20
=8(x1+x2)+23
=-24+23
=-1x1+x2=-3为什么韦达定理