Chapter 11 - Section 11.4 - Partial Sums of Arithmetic and Geometric Sequences - Exercise Set - Page 658: 24

$S_{12}=-696$

Using $S_{n}=\dfrac{n}{2}(2a_1+(n-1)d)$, the sum of the first $ 12 $ terms of the sequence $ -3,-13,-23,...,-113 $ is \begin{array}{l} S_{12}=\dfrac{12}{2}(2(-3)+(12-1)(-10))\\\\ S_{12}=6(-6-110)\\\\ S_{12}=6(-116)\\\\ S_{12}=-696 .\end{array}