Chapter 11 - Section 11.4 - Partial Sums of Arithmetic and Geometric Sequences - Exercise Set - Page 659: 30

$S_{15}=-495$

Using $S_{n}=\dfrac{n}{2}(2a_1+(n-1)d)$, the sum of the first $ 15 $ terms of the sequence $ -5,-9,-13,...,-61 $ is \begin{array}{l} S_{15}=\dfrac{15}{2}\left(2(-5)+(15-1)(-4)\right)\\\\ S_{15}=\dfrac{15}{2}\left(-10+-56\right)\\\\ S_{15}=\dfrac{15}{2}\left(-66\right)\\\\ S_{15}=-495 .\end{array}