Results 1 to 3 of 3

Math Help - Proving an identity.

  1. #1
    Newbie
    Joined
    Jun 2010
    Posts
    6

    Proving an identity.

    Show that Sin2A - tanA = (tanA).(cos2A) ??

    I have so far worked it to :

    Sin2A - tanA = - sinA/cosA + (2SinA).(cosA)

    Really stuck now any help would be appreciated guys.
    Last edited by mr fantastic; June 21st 2010 at 02:07 PM. Reason: Re-titled.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor
    Prove It's Avatar
    Joined
    Aug 2008
    Posts
    11,318
    Thanks
    1231
    \sin{2A} - \tan{A} = 2\sin{A}\cos{A} - \frac{\sin{A}}{\cos{A}}

     = \frac{2\sin{A}\cos^2{A}}{\cos{A}} - \frac{\sin{A}}{\cos{A}}

     = \frac{2\sin{A}\cos^2{A} - \sin{A}}{\cos{A}}

     = \frac{\sin{A}(2\cos^2{A} - 1)}{\cos{A}}

     = \left(\frac{\sin{A}}{\cos{A}}\right)(2\cos^2{A} - 1)

     = \tan{A}\cos{2A}.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Newbie
    Joined
    Jun 2010
    Posts
    6
    Many thanks,appreciated.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Proving an identity
    Posted in the Advanced Algebra Forum
    Replies: 1
    Last Post: October 12th 2009, 09:14 PM
  2. proving an identity
    Posted in the Trigonometry Forum
    Replies: 2
    Last Post: September 3rd 2009, 04:37 PM
  3. Proving an identity that's proving to be complex
    Posted in the Trigonometry Forum
    Replies: 1
    Last Post: July 21st 2009, 01:30 PM
  4. Proving an identity
    Posted in the Trigonometry Forum
    Replies: 3
    Last Post: July 2nd 2009, 09:08 PM
  5. Proving identity #1
    Posted in the Trigonometry Forum
    Replies: 2
    Last Post: April 9th 2009, 02:33 PM

Search Tags


/mathhelpforum @mathhelpforum