数学 Ⅰ¶ 1. 図形と計量¶ 1.1. 正弦定理¶ \[ \frac{a}{\sin{a}} = \frac{b}{\sin{b}} = \frac{c}{\sin{c}} = 2R \] 1.2. 余弦定理¶ \[ a^2 = b^2 + c^2 -2bc \cos{a} \] or \[ \cos{a} = \frac{b^2 + c^2 - a^2}{2bc} \]