cos(A+B)= cosAcosB- sinAsinB (1)cos(A-B)= cosAcosB+ sinAsinB (2)(2)-(1)cos(A-B)+ cos(A+B) = 2cosAcosBA+B= 3x/2A-B= x/2A= 2xB= xcos(x/2)+cos(3x/2)= [cos2x .cosx] /2