Question

The first term of a geometric series is 3, and the sum of the first term and the second term is 15. What is the sum of the first six terms?

Answer 1

Hello,

`u_(1)=3`

`u_(2)=3*r`


`u_(1)+u_(2)=15=3(1+r)`
==> 1+r=5==>r=4

3+3r+3r²+3r^3+...+3r^6=3(1+r+r²+...+r^6)=3*(r^7-1)/(r-1)=4^7-1=16383