Đáp án:
$\begin{array}{l}
A = \left| {x + 1} \right| + \left| {x + 3} \right|\\
+ Khi:x \ge - 1 \Rightarrow A = x + 1 + x + 3 = 2x + 4\\
+ Khi: - 3 \le x < - 1\\
\Rightarrow A = - x - 1 + x + 3 = 2\\
+ Khi:x < - 3\\
\Rightarrow A = - x - 1 - x - 3 = - 2x - 4
\end{array}$