Đáp án:
$\begin{array}{l}
a)\left| x \right| < 20\\
\Rightarrow x \in {\rm{\{ }} - 19; - 18;...; - 1;0;1;...;18;19\} \\
\Rightarrow S = \left( { - 19} \right) + \left( { - 18} \right) + ... + \left( { - 1} \right) + 0 + 1 + ... + 18 + 19\\
\Rightarrow S = 0\\
b)\left( {x + 13} \right) = \left( {x + 4} \right) + 9\\
Do:\left\{ {x + 4 \vdots x + 4} \right.\\
Nên\,\left( {x + 13} \right) \vdots x + 4 \Rightarrow 9 \vdots x + 4\\
\Rightarrow \left( {x + 4} \right) \in Ư\left( 9 \right) = {\rm{\{ }} - 9; - 3; - 1;1;3;9\} \\
\Rightarrow x \in {\rm{\{ }} - 13; - 7; - 5; - 3; - 1;5{\rm{\} }}\\
c)\\
C = \left\{ {{{2.3}^2} + \left[ {144 - {5^2} - \left( {15 - 169:13} \right)} \right]} \right\} - {2018^0}\\
= \left\{ {2.9 + \left[ {144 - 25 - \left( {15 - 13} \right)} \right]} \right\} - 1\\
= \left[ {18 + \left( {119 - 2} \right)} \right] - 1\\
= 18 + 117 - 1\\
= 134
\end{array}$
