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June 15th, 2010, 07:33 AM  #1 
Member Joined: Jun 2010 From: USA, Texas Posts: 85 Thanks: 0  Product to Sum Info
This is just a small minor question. Problem: 5 cos8x cos12x  5[ .5 [cos(8x12x) + 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?? 
June 15th, 2010, 07:39 AM  #2 
Global Moderator Joined: Oct 2008 From: London, Ontario, Canada  The Forest City Posts: 7,407 Thanks: 859 Math Focus: Elementary mathematics and beyond  Re: Product to Sum Info
cos(4x) = cos(4x)

June 15th, 2010, 07:57 AM  #3 
Member Joined: Jun 2010 From: USA, Texas Posts: 85 Thanks: 0  Re: Product to Sum Info
Oh I see! Thanks :] 

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