A) \[2\cos x\cos 2x\cos 3x\]
B) \[4\sin x\,\cos 2x\cos 3x\]
C) \[4\cos x\cos 2x\cos 3x\]
D) None of these
Correct Answer: C
Solution :
\[1+\cos \,\,2x+\cos \,4x+\cos \,6x\] \[=(1+\cos \,6x)+(\cos \,2x+\cos \,4x)\] \[=2\,{{\cos }^{2}}3x+2\,\cos \,3x\,\cos x=2\,\cos \,3x\,(\cos \,3x+\cos \,x)\] \[=4\,\cos x\,\cos \,2x\,\cos \,3x\].
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