a) Calcule $({\cos }^2\frac{\pi }{5}-{\mathrm{sen}}^2\frac{\pi }{5})\cos \frac{\pi }{10}-2\mathrm{sen}\frac{\pi }{5}\cos \frac{\pi }{5}\mathrm{sen}\frac{\pi }{10}$. 

b) Usando o resultado do item anterior, calcule $\mathrm{sen}\frac{\pi }{10}\cos \frac{\pi }{5}$.