+28 What is the sum of the first 7 terms of the series −8+16−32+64−...?
+118687 thanks Bliss, that is a nice straight forward way to approach the problem.
Here is a different way
This is a GP (Geometric progression)
the first term is -8
the common ratio r= 16/-8 = -2
\(S_n=\frac{a(r^n-1)}{r-1}\\~\\ S_7=\frac{-8((-2)^7-1)}{-2-1}\\~\\ S_7=\frac{-8(-128-1)}{-3}\\~\\ S_7=\frac{-8(-129)}{-3}\\~\\ S_7=\frac{-8(43)}{1}\\~\\ S_7=-344\\~\\\)
