三角形の決定
a sin  α = b sin  β = c sin  γ {\displaystyle {\frac {a}{\sin \alpha }}={\frac {b}{\sin \beta }}={\frac {c}{\sin \gamma }}}
α + β + γ = 180 ∘ {\displaystyle \alpha +\beta +\gamma =180^{\circ }}