In any triangle ABC if a , b ,c be lenghts of sides of triangle ABC then cosines of angles is given by the following formulae
\[\cos A =\frac{b^{2}+c^{2}-a^{2}}{2bc} \]
Boundary condition -none of a, b or c can be zero . This is true for Cos A Cos B and Cos C formula
Similarly
\[\cos B =\frac{a^{2}+c^{2}-b^{2}}{2ac} \]
\[\cos C =\frac{a^{2}+b^{2}-c^{2}}{2ab} \]
