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