cos A = $\frac{b^2 + c^2 - a^2}{(2bc)}$. Use when given SAS (two sides and included angle) or SSS (all three sides). It generalizes the Pythagorean theorem — when A = 90, cos A = 0 and it reduces to $a^2$ = $b^2$ + $c^2$. The sign of cos A tells you if angle A is acute (positive), right (zero), or obtuse (negative).