Question

3 (cos 2a cosa-sin 2a sin a )-7/2 cos 3a, при а= пи/6

Answer 1

$3(cos2a \; cosa-sin2a \; sina)-\frac{7}{2} \; cos3a=\\\\=3cos3a-\frac{7cos3a}{2}=\\\\=-\frac{cos3a}{2} =\\\\=-\frac{cos(3 \cdot \frac{\pi}{6}) }{2} =\\\\=-\frac{cos\frac{\pi}{2} }{2} =\\\\=-\frac{0}{2} = 0$