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Posted by **Rin** on Tuesday, December 13, 2011 at 6:03pm.

cos3x = 4cos^3x - 3cosx

- trig -
**Reiny**, Tuesday, December 13, 2011 at 6:23pmcos(3x)

= cos(2x + x)

= cos2xcosx - sin2xsinx

= (2cos^2 x -1)cosx - (2sinxcosx)sinx

= 2cos^3 x - cosx - 2sin^2 x (cosx)

= 2 cos^3x - cosx - 2(1 - cos^2 x)(cosx)

= 2 cos^3x - cosx - 2cosx + 2cos^3x

= 4cos^3x - 3cosx

= RS

Nice!

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