Tính:

$co{s^2}\frac{\pi }{8} + co{s^2}\frac{{3\pi }}{8}$

Ta có:

$co{s^2}\frac{\pi }{8} + co{s^2}\frac{{3\pi }}{8} = co{s^2}\frac{\pi }{8} + {\sin ^2}\left( {\frac{\pi }{2} - \frac{{3\pi }}{8}} \right)$

$= co{s^2}\frac{\pi }{8} + {\sin ^2}\frac{\pi }{8} = 1$.