第11章 11.12

$\tan\theta = 2 \qquad \frac{\sin\theta}{\cos\theta} = \frac{2}{1} = \frac{\frac{2}{r}}{\frac{1}{r}}$

$r = \sqrt{2^2 + 1^2} = \sqrt{5}$

これから

$\cos\theta = \frac{1}{\sqrt{5}} \qquad \sin\theta = \frac{2}{\sqrt{5}}$

$\cos^2\theta = \frac{1}{5} \qquad \sin\theta\cos\theta = \frac{2}{5}$