Chapter 11 - Section 11.4 - Partial Sums of Arithmetic and Geometric Sequences - Exercise Set - Page 658: 26

$S_{5}=-242$

Using $S_{n}=\dfrac{a_1(1-r^n)}{1-r}$, the sum of the first $ 5 $ terms of the sequence $ -2,-6,-18,... $ is \begin{array}{l} S_{5}=\dfrac{-2\left(1-\left( 3 \right)^5\right)}{1-3}\\\\ S_{5}=\dfrac{-2\left(-242\right)}{-2}\\\\ S_{5}=-242 .\end{array}