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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 sinysin(x - y) = sinx cosy - cosx siny cos(x + y) = cosx cosy - sinx siny cos(x - y) = cosx cosy + sinx sinytan(x + y) = (tanx + tany)/(1- tanx tany) tan(x - y) = (tanx - tany)/(1 + tanx tany)
