Bonjour;
[tex]f(iy)=\dfrac{i(iy)-1}{(iy+1)^2}=\dfrac{-y-1}{-y^2+1+2iy}=iy;\\\\\\\Rightarrow -y-1=iy(-y^2+1)-2y^2\Rightarrow2y^2-y-1=iy(1-y^2)\\\\\\\Rightarrow\left\{\begin{matrix}y(1-y^2)=0\\\\\\ 2y^2-y-1=0\end{matrix}\\\\\\\Rightarrow\left\{\begin{matrix}y=0\ ou\ 1-y^2=0\\\\\\ 2y^2-y-1=0\end{matrix}\\\\\\\Rightarrow\left\{\begin{matrix}y=0\ ou\ y=1\ ou\ y=-1\\\\\\ 2y^2-y-1=0\end{matrix}\\\\\\\\\\\Rightarrow y=1[/tex]
