$$\eqalign{
& Xet\,\,{f^2}\left( x \right) - 2f\left( x \right) = 0 \cr
& \Leftrightarrow \left[ \matrix{
f\left( x \right) = 0\,\,\left( 1 \right) \hfill \cr
f\left( x \right) = 2\,\,\left( 2 \right) \hfill \cr} \right. \cr
& \left( 1 \right) \Leftrightarrow \left[ \matrix{
x = - 2 \hfill \cr
x = 1\,\,\left( {nghiem\,\,boi\,\,2} \right)\,\,\left( {Triet\,\,tieu\,\,het\,\,voi\,\,tu} \right) \hfill \cr} \right. \cr
& \left( 2 \right) \Leftrightarrow \left[ \matrix{
x = {x_1} \in \left( { - 2; - 1} \right) \hfill \cr
x = 0 \hfill \cr
x = {x_2} \in \left( {1;2} \right) \hfill \cr} \right. \cr
& \Rightarrow DTHS\,\,g\left( x \right)\,\,co\,\,4\,\,TCD. \cr} $$
