分类：教育/科学-学习帮助
问题：高二数学数列题，救救我，明天要交等比数列{An}中，S3=7,S 
等比数列{An}中，S3=7,S6=63,求S9祝大家中秋
回答：S3=7,S6=63 ====>q≠-1
S3 =A1(1-q^3)/(1-q)=7
S6=A1(1-q^6)/(1-q)=63
S6/S3 =(1-q^6)/(1-q^3) =1+q^3=9 ====>q=2
==>
S3 =A1(1-8)/(1-2) =7A1=7  ===>A1=1
所以
S9=A1(1-q^9)/(1-q) =2^9-1 =511