
Trigonometry Trigonometry Math Forum 
 LinkBack  Thread Tools  Display Modes 
August 12th, 2012, 11:34 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, 11:39 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, 11:52 AM  #3 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,208 Thanks: 516 Math Focus: Calculus/ODEs  Re: Another trigonometric identity 
August 12th, 2012, 01:27 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, 01:44 PM  #5 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,208 Thanks: 516 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, 01:45 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, 01:52 PM  #7 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,208 Thanks: 516 Math Focus: Calculus/ODEs  Re: Another trigonometric identity 
August 12th, 2012, 02:03 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, 02:06 PM  #9 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,208 Thanks: 516 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, 02:19 PM  #10 
Senior Member Joined: Jul 2012 Posts: 225 Thanks: 0  Re: Another trigonometric identity
Thank you very much.


Tags 
identity, trigonometric 
Thread Tools  
Display Modes  

Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
trigonometric identity  matemagia  Trigonometry  4  August 30th, 2012 07:40 PM 
trigonometric identity  afrim  Trigonometry  8  October 1st, 2011 02:59 AM 
Help on trigonometric identity  chnixi  Algebra  1  August 19th, 2009 04:52 AM 
Trigonometric identity  munkiemunkie007  Algebra  2  May 4th, 2009 05:50 PM 
trigonometric identity  TG  Algebra  2  March 7th, 2008 04:22 PM 