x=x1代入方程：x1²+3x+1=0x1²=-3x1-1由韦达定理得x1+x2=-3 x1³+8x2+20=x1(-3x1-1)+8x2+20=-3x1²-x1+8x2+20=-3(-3x1-1)-x1+8x2+20=8x1+8x2+23=8(x1+x2)+23=8×(-3)+23=-1提示：反复对x1的高次方...