Помогитевычислитеcos 7π/20 cos 3π/20-sin π/25 sin 4π/15помогите

Решите задачу:

$cos\frac{7\pi}{20}*cos\frac{3\pi}{20}-sin\frac{\pi}{15}*sin\frac{4\pi}{15}\\\\ cos\frac{7\pi}{20}*cos\frac{3\pi}{20} = \frac{cos\frac{\pi}{2}+cos\frac{\pi}{5}}{2}\\\\ sin\frac{\pi}{15}*sin\frac{4\pi}{15}=\frac{cos\frac{\pi}{5}-cos\frac{\pi}{3}}{2}\\\\\ \frac{cos\frac{\pi}{5}-cos\frac{\pi}{5}+cos\frac{\pi}{3}}{2}=\frac{\frac{1}{2}}{2}=\frac{1}{4}$