Question

等比数列{an}的前三项为a,1/2(a+1),1/3(a+1),求次数列的和S5

Answer 1

[1/2(a+1)]^2=1/3(a+1)*a 得：a=3,q=2/3 a5=3*(2/3)^4=16/27

s5=[3-(16/27)*2/3]/(1-2/3)=8*(22/27)