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

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

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

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