The trigonometric functions of the sum or difference of two angles that are expressed in terms of separate functions of the angles.
sin(x + y) = sinx cosy + cosx siny
sin(x - y) = sinx cosy - cosx siny
cos(x + y) = cosx cosy - sinx siny
cos(x - y) = cosx cosy + sinx siny
tan(x + y) = (tanx + tany)/(1- tanx tany)
tan(x - y) = (tanx - tany)/(1 + tanx tany)