Question

a1=-2.7,q=-1/3,an=1/90,求相应的等比数列{an}的前n项和Sn,用高中知识,

Answer 1

1/90=an=a1q^(n-1)=-2.7x(-1/3)^(n-1) (-1/3)^(n-1)=(1/90)/(-2.7)=-1/243=(-1/3)^5 n-1=5,解得：n=6 那么,等比数列S6 =a1x(1-q^6)/(1-q) =(-2.7)x[1-(-1/3)^6]/[1-(-1/3)] =-91/45