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August 12th, 2012, 10:34 AM  #1 
Senior Member Joined: Jul 2012 Posts: 225 Thanks: 0  Another trigonometric identity
Can anyone help me prove this http://s137.photobucket.com/albums/q225 ... trigo4.jpg please?

August 12th, 2012, 10:39 AM  #2 
Senior Member Joined: Jul 2012 Posts: 225 Thanks: 0  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.

August 12th, 2012, 10:52 AM  #3 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,204 Thanks: 511 Math Focus: Calculus/ODEs  Re: Another trigonometric identity 
August 12th, 2012, 12:27 PM  #4 
Senior Member Joined: Jul 2012 Posts: 225 Thanks: 0  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. 
August 12th, 2012, 12:44 PM  #5 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,204 Thanks: 511 Math Focus: Calculus/ODEs  Re: Another trigonometric identity
Try using a doubleangle identity for cosine on then factor the resulting quadratics. Then use doubleangle identities for cosine again.

August 12th, 2012, 12:45 PM  #6 
Senior Member Joined: Jul 2012 Posts: 225 Thanks: 0  Re: Another trigonometric identity
I did, tried that several times, didn't get tan^4x.

August 12th, 2012, 12:52 PM  #7 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,204 Thanks: 511 Math Focus: Calculus/ODEs  Re: Another trigonometric identity 
August 12th, 2012, 01:03 PM  #8 
Senior Member Joined: Jul 2012 Posts: 225 Thanks: 0  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.

August 12th, 2012, 01:06 PM  #9 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,204 Thanks: 511 Math Focus: Calculus/ODEs  Re: Another trigonometric identity
There are several doubleangle identities for cosine, so the second time, I used the forms that would eliminate the plus or minus one.

August 12th, 2012, 01:19 PM  #10 
Senior Member Joined: Jul 2012 Posts: 225 Thanks: 0  Re: Another trigonometric identity
Thank you very much.


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