Results 1 to 4 of 4

Math Help - Trig identity question!

  1. #1
    LHS
    LHS is offline
    Member
    Joined
    Feb 2009
    From
    Oxford
    Posts
    84

    Lightbulb Trig identity question!



    Another question I have unfortunately drawn a blank on..
    Any help is greatly appreciated!
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor red_dog's Avatar
    Joined
    Jun 2007
    From
    Medgidia, Romania
    Posts
    1,252
    Thanks
    5
    \sin x(\cos y+2\sin y)=\cos x(2\cos y-\sin y)\Leftrightarrow

    \Leftrightarrow\sin x\cos y+2\sin x\sin y=2\cos x\cos y-\sin y\cos x\Leftrightarrow

    \Leftrightarrow \sin x\cos y+\sin y\cos x=2(\cos x\cos y-\sin x\sin y)\Leftrightarrow

    \Leftrightarrow\sin(x+y)=2\cos(x+y)\Leftrightarrow  \tan(x+y)=2
    Follow Math Help Forum on Facebook and Google+

  3. #3
    MHF Contributor
    Joined
    Sep 2008
    From
    West Malaysia
    Posts
    1,261

    Re :

     \sinx\cosy+2 \sinx\siny=2 \cosx\siny-\cosx\siny

     \sinx\cosy+\cosx\siny=2(\cosx\cosy-\sinx\siny)

     \sin(x+y)=2cos(x+y)

     \frac{\sin(x+y)}{cos(x+y)}=2

     \tan(x+y)=2

    Sorry about some confusion .. Can someone pls check my latex .
    Follow Math Help Forum on Facebook and Google+

  4. #4
    LHS
    LHS is offline
    Member
    Joined
    Feb 2009
    From
    Oxford
    Posts
    84
    Ah I see now, I tried expanding it, but I see now you can swap them around so they fit into the sin and cos addition formulae.
    Problem with these questions as its often very hard if you don't see the method you need to use at the beginning.

    Cheers!
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. trig identity question
    Posted in the Trigonometry Forum
    Replies: 2
    Last Post: August 13th 2010, 06:30 PM
  2. Trig Identity math question.
    Posted in the Trigonometry Forum
    Replies: 8
    Last Post: July 11th 2010, 07:19 PM
  3. another trig identity question
    Posted in the Trigonometry Forum
    Replies: 2
    Last Post: April 9th 2010, 09:41 PM
  4. Trig identity question
    Posted in the Trigonometry Forum
    Replies: 2
    Last Post: May 14th 2009, 12:33 PM
  5. Little trig identity question.
    Posted in the Trigonometry Forum
    Replies: 1
    Last Post: September 17th 2008, 02:06 PM

Search Tags


/mathhelpforum @mathhelpforum