第1个回答 2013-10-16
1. 等比数列,有a(n+2)=4*an a1=a3/4 a2=a1/2 ∴ a1*a2*a3=(a3)^3/8=(a3/2)^3 a4=a6/4 a5=a6/2 ∴ a4*a5*a6=(a6)^3/8=(a6/2)^3 …… ∴ a1*a2*a3*…a30=(a3/2)^3*(a6/2)^3*…(a30/2)^3=(a3*a6*a9*...a30)^3*(1/2)^30=2^30 ∴ a3*a6*a9*...a30=2^202. q^3=1-1/4=0.75 q=0.75^(1/3) an=640 a(n+9)=an*q^9=640*0.75^3=270