Cosine law

#basis

余弦定理是描述三角形中三边长度与一个角的余弦值关系的数学定理，是勾股定理在一般三角形情形下的推广，勾股定理是余弦定理的特例

\begin{array}{l} a^{2} = b^{2} + c^{2} - 2 b c c o s A \\ b^{2} = a^{2} + c^{2} - 2 a c c o s B \\ c^{2} = a^{2} + b^{2} - 2 a b c o s C \\ c o s A = \frac{- a^{2} + b^{2} + c^{2}}{2 b c} \\ c o s B = \frac{a^{2} - b^{2} + c^{2}}{2 a c} \\ c o s C = \frac{a^{2} + b^{2} - c^{2}}{2 a b} \end{array}