[Tanius]

\( {\dfrac{1}{2}\left(\dfrac{1-\sen (x)}{1+\sen (x)}\right)^{- 1/2}\left(\dfrac{-2\cos (x)}{(1+\sen (x))^2}\right)=\dfrac{1}{2\sqrt{\dfrac{1-\sen (x)}{1+\sen (x)}}} \left(\dfrac{-2\cos (x)}{(1+\sen (x))^2}\right)}} \)

[Tanius]

\( {\dfrac{1}{2\sqrt{\dfrac{1-\sen (x)}{1+\sen (x)}}} \left(\dfrac{-2\cos (x)}{(1+\sen (x))^2}\right)}} = \dfrac{1}{\sqrt{\dfrac{1-\sen (x)}{1+\sen (x)}}}\cdot\dfrac{1}{2}\left(\dfrac{-2\cos (x)}{(1+\sen (x))^2}\right)}} = \dfrac{1}{\sqrt{\dfrac{1-\sen (x)}{1+\sen (x)}}}\left(\dfrac{-\cos (x)}{(1+\sen (x))^2}\right)}}} \)

[Tanius]

\( {\dfrac{1}{2\sqrt{\dfrac{1-\sen (x)}{1+\sen (x)}}} =\dfrac{1}{2}\cdot \dfrac{1}{\sqrt{\dfrac{1-\sen (x)}{1+\sen (x)}}} \)