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June 15th, 2010, 07:33 AM   #1
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Product to Sum Info

This is just a small minor question.

Problem: 5 cos8x cos12x
------
5[ .5 [cos(8x-12x) + cos(8x+12x)] ]
5[ .5 [cos(-4x) + cos(20x)] ]

But the answer is:
5/2(cos20x + cos4x)

Did I do something wrong? How did the -4 turn into +4??
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June 15th, 2010, 07:39 AM   #2
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Re: Product to Sum Info

cos(-4x) = cos(4x)
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June 15th, 2010, 07:57 AM   #3
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Re: Product to Sum Info

Oh I see!
Thanks :]
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