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,

Ok, i have seen the mistake the answer is 4095.


$u_{1}=3\\u_{2}=?\\u_{1}+u_{2}=15==>u_{2}=15-3=12=4*3$
$u_{1}+u_{2}+...u_{6}=3+3*4^1+3*4^2+...+3*4^5\\=3*(1+4+4^2+4^3+...+4^5)=3*\frac{4^6-1}{4-1}=3*(4^6-1)/3=4095$