x1和x2是方程x²+3x+1=0的两个实数根
∴x1²+3x1+1=0,x1+x2=-3
即 x1²=-1-3x1
x1³+8 x2+20
=x1×x1²+8x2+20
=x1×(-1-3x1)+8x2+20
=-x1-3x1²+8x2+20
=-x1-3×(-1-3x1)+8x2+20
=8x1+8x2+3+20
=8*x1+x2)+23
=8×(-3)+23
=-1-x1-3×(-1-3x1)+8x2+20这步怎么到8x1+8x2+3+20的、？-x1-3×(-1-3x1)+8x2+20
=-x1+3+9x1+8x2+20
=8x1+8x2+3+20
=8*x1+x2)+23
=8×(-3)+23
=-1