Question

Find the sum of the first 8 terms of the geometric sequence if the first term is 9 and the common ratio is -3.

Answer 1

hmmmm let's see  $\bf \textit{sum of a finite geometric sequence}=S_n=a_1\left( \cfrac{1-r^n}{1-r} \right) \\\\ \begin{cases} n=n^{th}\ term\\ a_1=\textit{first term}\\ r=\textit{common ratio} \end{cases}\qquad thus\implies S_8=9\left( \cfrac{1-(-3)^8}{1-(-3)} \right)$