Question

第十七题怎么做，求过程～谢谢啦

Answer 1

S2=a2q=a3
a3+S3=S2+S3=b1+b2+(b1+b2+b3)=2b1+2b2+b3=2b1+2(b 1+d)+(b1+2d)=5b1+4d=15+4d=27
(其中d为公差)
d=3
bn=b1+(n-1)d=3+3(n-1)=3n
a2q=a1q²=q²=S2=b1+b2=3+6=9
由于q＞0
所以q=3
an=a1q^(n-1)=3^(n-1)

追答

Sn=n(b1+bn)/2=n(3+3n)/2
2Sn=3n(n+1)
cn=9/2Sn=3/n(n+1)=3*[1/n-1/(n+1)]
Tn=3×[(1-1/2)+(1/2-1/3)+…+(1/n-1/(n+1))]
=3×[1-1/(n+1)]
=3n/(n+1)

Answer 2

Answer 3