덧셈정리 cos 값

$\displaystyle \sin ^{2} \theta = \frac{4}{5}$에서 $\displaystyle \sin \theta = \frac{2}{\sqrt{5}} , \cos \theta = \frac{1}{\sqrt{5}}$ $\displaystyle \cos \left( \theta + \frac{\pi}{4} \right) = \cos \theta \cos \frac{\pi}{4} - \sin \theta \sin \frac{\pi}{4}$ $\displaystyle = \frac{1}{\sqrt{5}} \frac{1}{\sqrt{2}} - \frac{2}{\sqrt{5}} \frac{1}{\sqrt{2}} = - \frac{1}{\sqrt{10}} = p$ $\displaystyle \therefore \frac{1}{p ^{2}} = 10$