MCQ
$1 + \cos 2x + \cos 4x + \cos 6x = $
- A$2\cos x\cos 2x\cos 3x$
- B$4\sin x\,\cos 2x\cos 3x$
- ✓$4\cos x\cos 2x\cos 3x$
- DNone of these
✓
Answer
Correct option: C.
$4\cos x\cos 2x\cos 3x$
c
(c) $1 + \cos \,\,2x + \cos \,4x + \cos \,6x$
(c) $1 + \cos \,\,2x + \cos \,4x + \cos \,6x$
$ = (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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