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September 16th, 2009, 09:49 AM   #1
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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.
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September 16th, 2009, 04:42 PM   #2
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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.
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September 17th, 2009, 03:01 AM   #3
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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?
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September 17th, 2009, 05:25 AM   #4
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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.
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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
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Yes, but omit the redundant "=" at the end of a line when the next line starts with "=".
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