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 September 16th, 2009, 09:49 AM #1 Newbie   Joined: Sep 2009 Posts: 3 Thanks: 0 trigonometry Write 2sin²5xcos3x as a sum of cosinus and/or sinusterms. Use Eulers formula if you can. Also control your answer by putting x=0 before and after. I can't even start with x=0 in this one is it 2sin²5*0cos3*0 = 0 ? Please show me every step in how you do it, because this is the hardest thing to me to understand.
 September 16th, 2009, 04:42 PM #2 Global Moderator   Joined: May 2007 Posts: 6,807 Thanks: 716 Re: trigonometry For your last question, sinx = 0 when x = 0. So any expression multiplied by sinax, where a is any constant, will be 0.
 September 17th, 2009, 03:01 AM #3 Newbie   Joined: Sep 2009 Posts: 3 Thanks: 0 Re: trigonometry well, certainly doesn't help me much though Doesn't even know how to start, should i rewrite the cosinepart of the equation, or sine? And how?
 September 17th, 2009, 05:25 AM #4 Global Moderator   Joined: Dec 2006 Posts: 20,931 Thanks: 2205 One way to do it is to replace 2sin²(5x) with 1 - cos(10x). That gives you (1 - cos(10x))cos(3x), i.e., cos(3x) - cos(10x)cos(3x). Now cos(10x)cos(3x) can be replaced with (cos(13x) + cos(7x))/2.
September 17th, 2009, 07:37 AM   #5
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Re: trigonometry

Solved it, and controlled my answer with yours to be sure.

This looks right?
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 September 18th, 2009, 07:17 AM #6 Global Moderator   Joined: Dec 2006 Posts: 20,931 Thanks: 2205 Yes, but omit the redundant "=" at the end of a line when the next line starts with "=".

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