Demostrar que:
$$ \sin \theta \sin 2\theta + \sin 2\theta \sin 3\theta + \sin 3\theta \sin 4\theta + \dots \text{ hasta } n \text{ términos} $$
$$ = \frac{n}{2} \cos \theta - \frac{1}{2} \frac{\sin n\theta}{\sin \theta} \cos (n+2)\theta $$