`A=|x-2018|-|x-2017|`
`⇒|x-2018|-|x-2017|<=|(x-2018)-(2018)|`
`⇒|1|=1`
Dấu "=" xảy ra ⇔ $\left\{\begin{matrix} (x-2018)(x-2017) & & \\ x-2018\leq x-2017 & & \end{matrix}\right.$ ⇒ \(\left[ \begin{array}{l}x≥2018\\x≤2017\end{array} \right.\)
Vậy GTLN của `A` là `1`
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