Làm theo công thức tổng quát:

a) \(\left|x\right|< m\Leftrightarrow-m\le x\le m\) (Tự làm)

b) \(\left|x\right|>m\Leftrightarrow\left[{}\begin{matrix}x>m\\x< -m\end{matrix}\right.\)

c) \(\dfrac{2}{5}< \left|x-\dfrac{7}{5}\right|< \dfrac{3}{5}\)

\(\Rightarrow\left\{{}\begin{matrix}\left|x-\dfrac{7}{5}\right|>\dfrac{2}{5}\\\left|x-\dfrac{7}{5}\right|< \dfrac{3}{5}\end{matrix}\right.\)

+) TH1: \(\left|x-\dfrac{7}{5}\right|>\dfrac{2}{5}\Rightarrow\left[{}\begin{matrix}x-\dfrac{7}{5}>\dfrac{2}{5}\\x-\dfrac{7}{5}< -\dfrac{2}{5}\end{matrix}\right.\)

\(\Rightarrow\left[{}\begin{matrix}x>\dfrac{9}{5}\\x< 1\end{matrix}\right.\)

\(\Rightarrow\dfrac{9}{5}< x< 1\) (vô lí)

+) TH2: \(\left|x-\dfrac{7}{5}\right|< \dfrac{3}{5}\Rightarrow\dfrac{-3}{5}< x-\dfrac{7}{5}< \dfrac{3}{5}\)

Với \(x-\dfrac{7}{5}>\dfrac{-3}{5}\Rightarrow x>\dfrac{4}{5}\)

Với \(x-\dfrac{7}{5}< \dfrac{3}{5}\Rightarrow x< 2\)

Khi đó: \(\dfrac{4}{5}< x< 2\) (nhận)

Vậy \(\dfrac{4}{5}< x< 2.\)