`| 1 - 2x | = a`
`⇔ `\(\left[ \begin{array}{l}1 - 2x = a\\1 - 2x = - a\end{array} \right.\)
`⇔` \(\left[ \begin{array}{l}2x = 1 - a\\2x = 1 - ( - a )\end{array} \right.\)
`⇔` \(\left[ \begin{array}{l}x = ( 1 - a ) : 2\\x = [ 1 - ( - a ) ] : 2\end{array} \right.\)
Vậy , `x ∈ { ( 1 - a ) : 2 ; [ 1 - ( - a ) ] : 2 }`