Chọn khẳng định đúng trong các khẳng định sau:
\(\left|f\left(x\right)\right|\le a\Leftrightarrow-a\le f\left(x\right)\le a,\forall a>0\).\(\left|f\left(x\right)\right|\le a\Leftrightarrow-a\le f\left(x\right)\le a,\forall a\in R\).\(\left|f\left(x\right)\right|\le a\Leftrightarrow\left[{}\begin{matrix}f\left(x\right)\le-a\\f\left(x\right)\ge a\end{matrix}\right.,\forall a>0\).\(\left|f\left(x\right)\right|\le a\Leftrightarrow\left[{}\begin{matrix}f\left(x\right)\le-a\\f\left(x\right)\ge a\end{matrix}\right.,\forall a\in R\).