Вычислите cos 7π/36*cos 5π/36-sin π/9*sin π/18.

\[\cos\frac{7\pi}{36} \cdot \cos\frac{5\pi}{36} - \sin\frac{\pi}{9} \cdot \sin\frac{\pi}{18} =\]

\[= \frac{1}{4} + \frac{1}{2}\cos\frac{\pi}{18} - \frac{1}{2}\cos\frac{\pi}{18} + \frac{\sqrt{3}}{4} =\]

\[= \frac{1}{4} + \frac{\sqrt{3}}{4} = \frac{1 + \sqrt{3}}{4}\]