Question

cosacos a/2+ sinasin a/2

Answer 1

$\cos( \alpha ) \times \cos( \frac{ \alpha }{2} ) + \sin( \alpha ) \times \sin( \frac{ \alpha }{2} ) = \cos( \alpha - \frac{ \alpha }{2} ) = \cos( \frac{1}{2} \alpha )$