Cos 3п/10×cos 2п/5 решите пожалуйста

Решите задачу:

$cos\frac{3\pi}{10}=cos54^\circ =sin36^\circ=\frac{\sqrt{5-\sqrt5}}{2\sqrt2}\; ;\\\\cos\frac{2\pi}{5}=cos72^\circ=sin18^\circ =\frac{\sqrt5-1}{4}\\\\\\cos\ffrac{3\pi}{10}\cdot cos\frac{2\pi}{5}=\frac{\sqrt{5-\sqrt5}}{2\sqrt2}\cdot \frac{\sqrt5-1}{4} =\frac{\sqrt{(5-\sqrt5)(\sqrt5-1)^2}}{8\sqrt2}=\\\\= \frac{\sqrt{\sqrt5(\sqrt5-1)(\sqrt5-1)^2}}{8\sqrt2} =\frac{\sqrt{\sqrt5(\sqrt5-1)^3}}{8\sqrt2}=\frac{\sqrt{40-16\sqrt5}}{8\sqrt2}$