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August 12th, 2012, 10:34 AM   #1
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Another trigonometric identity

Can anyone help me prove this http://s137.photobucket.com/albums/q225 ... trigo4.jpg please?
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August 12th, 2012, 10:39 AM   #2
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Re: Another trigonometric identity

I should mention - you're not allowed to use the tanx+tany identity. You must solve by representing tanx as sinx/cosx.
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August 12th, 2012, 10:52 AM   #3
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Re: Another trigonometric identity





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August 12th, 2012, 12:27 PM   #4
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Re: Another trigonometric identity

I so fail at this subject, donno what I'm gonna do.

http://s137.photobucket.com/albums/q225 ... trigo5.jpg

Can't solve this now. Can't solve 2 problems in a row.
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August 12th, 2012, 12:44 PM   #5
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Re: Another trigonometric identity

Try using a double-angle identity for cosine on then factor the resulting quadratics. Then use double-angle identities for cosine again.
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August 12th, 2012, 12:45 PM   #6
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Re: Another trigonometric identity

I did, tried that several times, didn't get tan^4x.
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August 12th, 2012, 12:52 PM   #7
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August 12th, 2012, 01:03 PM   #8
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Re: Another trigonometric identity

Man, you are the best. I have no words. Just one question - I did the same as you did, double angle identity on cosine4? but how did you know not to use that same identity on cos2?? because I did, and it made the problem more complicated.
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August 12th, 2012, 01:06 PM   #9
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Re: Another trigonometric identity

There are several double-angle identities for cosine, so the second time, I used the forms that would eliminate the plus or minus one.
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August 12th, 2012, 01:19 PM   #10
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Re: Another trigonometric identity

Thank you very much.
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