Answer:
[tex]S_7=6554[/tex]
Step-by-step explanation:
[tex]\boxed{\begin{minipage}{7 cm}\underline{Sum of the first $n$ terms of a geometric series}\\\\$S_n=\dfrac{a(1-r^n)}{1-r}$\\\\where:\\\phantom{ww}$\bullet$ $a$ is the first term. \\ \phantom{ww}$\bullet$ $r$ is the common ratio.\\\end{minipage}}[/tex]
Given geometric series:
[tex]2-8+32-...+a_n[/tex]
The first term (a) of the sequence is 2:
[tex]\implies a=2[/tex]
To find the common ratio (r), divide consecutive terms:
[tex]\implies r=\dfrac{a_2}{a_1}=\dfrac{-8}{2}=-4[/tex]
To find the sum of the first 7 terms, substitute the found values of a and r into the formula, along with n = 7:
[tex]\implies S_7=\dfrac{2(1-(-4)^7)}{1-(-4)}[/tex]
[tex]\implies S_7=\dfrac{2(1-(-16384))}{5}[/tex]
[tex]\implies S_7=\dfrac{2(16385)}{5}[/tex]
[tex]\implies S_7=\dfrac{32770}{5}[/tex]
[tex]\implies S_7=6554[/tex]
