an =√(a(n-1)logan = (1/2)loga(n-1){logan} 是等比数列,q=1/2logan = (1/2)^(n-1) .(loga1)= (1/2)^(n-1)log(a1.a2.an)=loga1+loga2+...+logan=1+1/2+.+(1/2)^(n-1)= 2[1-(1/2)^n]