Cos(3pi/5) + cos(pi/5) помогите, его нужно решить, получается число в ответе, знаю точно

 [latex] cos(\frac{3\pi}{5}) + cos(\frac{\pi}{5}) = 2cos\frac{\pi}{5}*cos\frac{2\pi}{5} =\\ \frac{2cos \frac{\pi}{5}*cos\frac{2\pi}{5}*sin\frac{\pi}{5}}{sin\frac{\pi}{5}} =\\ [/latex]        [latex] \frac{2cos \frac{\pi}{5}*cos\frac{2\pi}{5}*sin\frac{\pi}{5}}{sin\frac{\pi}{5}} =\\ \frac{ sin\frac{2\pi}{5}*cos\frac{2\pi}{5}}{ sin\frac{\pi}{5}} = \frac{sin\frac{4\pi}{5}}{2*sin\frac{\pi}{5}} =\frac{1}{2} \\ sin\frac{4\pi}{5}=sin\frac{\pi}{5}\\ [/latex]