$\sin \alpha+\sin \beta=2$ sin $ \frac{\alpha+\beta}{2}\cdot $ cos $ \frac{\alpha-\beta}{2}$
$\sin \alpha-\sin \beta=2$ cos $ \frac{\alpha+\beta}{2}\cdot $ sin $ \frac{\alpha-\beta}{2}$
$\cos \alpha+\cos \beta=2$ cos $ \frac{\alpha+\beta}{2}\cdot $ cos $ \frac{\alpha-\beta}{2}$
$\cos \alpha-\cos \beta=-2$ sin $ \frac{\alpha+\beta}{2}\cdot $ sin $ \frac{\alpha-\beta}{2}$
b) $\displaystyle \frac{\sin 10°+\sin 20°}{2}=\sin 15°\cos 5°$ p
c) $\cos 300°-\cos (-100°)=-2\sin 100°\sin 200°$ p
a) $\sin \frac{\pi}{7}+\sin \frac{\pi}{9}\qquad $ b) $\cos \frac{2\pi}{5}-\cos \frac{\pi}{10}$
a) $\tan \frac{\pi}{12}-\tan\frac{5\pi}{12}\qquad $ b) $\displaystyle \frac{\cos 135°-\cos 45°}{\sin 15°-\cos 15°}$