====== 삼각함수 공식 ======
  - $$ \csc \ \theta = \frac{1}{\sin \ \theta} $$
  - $$ \sec \ \theta = \frac{1}{\cos \ \theta} $$
  - $$ \tan \ \theta = \frac{\sin \ \theta}{\cos \ \theta} $$
  - $$ \cot \ \theta = \frac{\cos \ \theta}{\sin \ \theta} $$
  - $$ \cot \ \theta = \frac{1}{\tan \ \theta} $$
  - $$ \sin^{2}\theta + \cos^{2}\theta = 1 $$
  - $$ 1 + \tan^{2}\theta = sec^{2}\theta $$
  - $$ 1 + \cot^{2}\theta = csc^{2}\theta $$
  - $$ \sin(-\theta) = -\sin \ \theta $$
  - $$ \cos(-\theta) = \cos \ \theta $$
  - $$ \tan(-\theta) = -\tan \ \theta $$
  - $$ \sin(\frac{\pi}{2} - \theta) = \cos \ \theta $$
  - $$ \cos(\frac{\pi}{2} - \theta) = \sin \ \theta $$
  - $$ \tan(\frac{\pi}{2} - \theta) = \cot \ \theta $$
  - $$ \sin(x + y) = \sin \ x \ \cos \ y + \cos \ x \ \sin \ y $$
  - $$ \sin(x - y) = \sin \ x \ \cos \ y - \cos \ x \ \sin \ y $$
  - $$ \cos(x + y) = \cos \ x \ \cos \ y - \sin \ x \ \sin \ y $$
  - $$ \cos(x - y) = \cos \ x \ \cos \ y + \sin \ x \ \sin \ y $$
  - $$ \tan(x + y) = \frac{\tan \ x + \tan \ y}{1 - \tan \ x \ \tan \ y} $$
  - $$ \tan(x - y) = \frac{\tan \ x - \tan \ y}{1 + \tan \ x \ \tan \ y} $$
  - $$ \sin \ 2x = 2\sin \ x \ \cos \ x $$
  - $$ \cos \ 2x = \cos^{2}x - \sin^{2}x = 2\cos^{2}x - 1 = 1 - 2\sin^{2}x $$
  - $$ \tan \ 2x = \frac{2\tan \ x}{1-\tan^{2}x} $$