cos A = (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).