a2=3,a5=a2*q^3=24,q^3=8a1=a2/q=3/2数列a1,a4,a7,a10...a3n-2..公比Q=q^3=8a3n-2=a1*Q^(n-1)=3/2*8^(n-1)
也成等比数列，首项为a1=3/2公比为q=8
