Please answer the question in the attachment.
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Answer:
cos x cos 2x cos 3x dx
==> (1/2) * (2 cos x * cos 3x) cos 2x
==> (1/2) * (cos 2x + cos 4x)
==> (1/4) * (2 cos²x + 2 cos 2x * cos 4x)
==> (1/4) * (1 + cos 4x + cos 2x + cos 6x)
Integrate it now.
==> (1/4) * ∫(1 + cos 4x + cos 2x + cos 6x) * dx
==> (1/4) * (x + sin4x/4 + sin2x/2 + sin6x/x) + c
==> (1/48) * (12x + 6 sin 2x + 3 sin 4x + 2 sin 6x) + c
Result,
Option(A).
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