Tienes :
\( \sin(x) = 2\cdot \sen(\dfrac{x}{2}) \cdot \cos(\dfrac{x}{2}) = \dfrac{2\cdot \sen(\dfrac{x}{2}) \cdot \cos(\dfrac{x}{2})}{1} = \dfrac{2\cdot \sen(\dfrac{x}{2}) \cdot \cos(\dfrac{x}{2})}{\sen^2(\dfrac{x}{2}) + \cos^2(\dfrac{x}{2})} \)
Divide numerador y denominador por \( \cos^2(\dfrac{x}{2}) \)