Question

cos13a-cos27a срочно

Answer 1

$\cos(13 \alpha ) - \cos(27 \alpha ) = - 2 \sin \frac{ \alpha + \beta }{2} \times \sin( \frac{ \alpha - \beta }{2} ) = - 2sin \frac{13 \alpha \ + 27 \alpha }{2} \times \ \ sin \frac{13 \alpha - 27 \alpha }{2} = - 2 \sin20 \alpha \times \ - sin( 7 \alpha ) = 2 \sin20 \alpha \times \sin7 \alpha$