Question

已知等比数列[An]中,A2=3 A5=81 1.求[An]的通向公式

Answer 1

1.

a5/a2=(a1•q^4)/(a1•q)=q³=81/3=27

∴q=3

a2=a1•q=3

∴a1=1

∴an=a1•q^(n-1)=3^(n-1)

2.

S6=b1+b2+...+b6

=3(a1+a2+...a6)-2×6

=3(1-3^6)/(-2)-12

=3(3³•3³-1)/2 -12

=1080