a+b=-3,ab=1
a,b是关于方程x^2+3x+1=0的两个解
a^2+3a+1=0
a^2=-3a-1
a^2+3a+1=0
a^2+3a=-1
a^3+8b
=a*a^2+8b
=a*(-3a-1)+8b
=-3a^2-a+8b
=-3a^2-9a+8a+8b
=-3(a^2+3a)+8(a+b)
=-3*(-1)+8*(-3)
=3-24
=-21