+ Xét $1-|x| \ge 0\Leftrightarrow -1 \le x \le 1.$ BPT
$\Leftrightarrow 1-|x|<a-x\Leftrightarrow \left[ {\begin{matrix} 1<a \text{ nếu } 0 \le x \le 1\\ x<\frac{a-1}{2} \text{ nếu } -1 \le x <0\end{matrix}} \right.$
+ Xét $1-|x|< 0\Leftrightarrow\left[ {\begin{matrix} x>1\\ x<-1\end{matrix}} \right.$ BPT
$\Leftrightarrow |x|-1<a-x\Leftrightarrow \left[ {\begin{matrix} x<\frac{a+1}{2} \text{ nếu } x>1\\ -1<a \text{ nếu } x<-1\end{matrix}} \right.$