a1+a2+a3=-3,a1*a2*a3=8
a2/q+a2+a2*q=-3
a2^3=8,a2=2
2/q+2+2q=-3
2q+5+2/q=0
2q^2+5q+2=0
q=-1/2或-2,a1=-4或-1
an=-4*(-1/2)^(n-1)或an=-(-2)^(n-1)
a5=-1/4或-16
a1*a3*a5*a7*a9
=a5^5
=-1/2^10或-2^20