Posted by **Scott** on Tuesday, February 9, 2010 at 2:29pm.

How do I simplify this?

cos4xcos3x-sin4xcos3x

I pulled the cos3x out as a common factor but that doesn't really help. Can someone please direct me on this. Thanks!

- Trig -
**drwls**, Tuesday, February 9, 2010 at 3:52pm
Are you sure is isn't

cos4x*cos3x - sin4x*sin3x ?

That would be cos 7x.

If not, then

cos4x*cos3x - sin4x*cos3x

= cos 3x (cos 4x - sin 4x)

= cos 3x [8 cos^2x -8cos^2 x +1 - 8 cos^3 x sinx + 4 cos x sin x)]

That is not a simplification.

There may have been a typo error in your problem assignment.

- Trig -
**Scott**, Tuesday, February 9, 2010 at 3:57pm
Thanks a lot. I also thought the question was written improperly but I spent a lot of time trying to simplify the darn thing! Thanks for your help.

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