Law of cosines
In trigonometry, the law of cosines (also known as the cosine formula or cosine rule or Al-Kashi’s theorem) relates the lengths of the sides of a triangle to the cosine of one of its angles. For a triangle with sides a {\displaystyle a} , b {\displaystyle b} , and c {\displaystyle c} , opposite respective angles α {\displaystyle \alpha } , β {\displaystyle \beta } , and γ {\displaystyle \gamma } (see Fig.