integration of cos3xcosx Watch

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Freeway
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Report Thread starter 13 years ago
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does anyone know how to do this?
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dvs
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Report 13 years ago
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Use the identitiy:
cos(A+B/2)cos(A-B/2) = (1/2)(cosA + cosB)
with A=4x and B=2x.

i.e.
cos3x cosx = cos(4x+2x/2)cos(4x-2x/2) = (1/2)cos4x + (1/2)cos2x

Integrating:
(1/8)sin4x + (1/4)sin2x + C
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