Fie functia \( f:\left[ -1;1\right] \rightarrow \mathbb{R}\),
\[ f\left( x\right) =\left\{
\begin{array}
[c]{c}%
\frac{\pi}{2}-2\operatorname{arctg}\sqrt{\frac{1-x}{1+x}},~~~x\in(-1;1]\\
-\frac{\pi}{2},~~x=-1
\end{array}
\right. \]
si
\[M=\left\{ \left. m\in%
%TCIMACRO{\U{211d} }%
%BeginExpansion
\mathbb{R}
%EndExpansion
\right\vert f\left( x\right) =mx\text{ are 3 solutii reale distincte}%
\right\} .\] Aflati \(M.\)