Results 1 to 4 of 4

Math Help - Prove Trig Identity

  1. #1
    Raj
    Raj is offline
    Junior Member
    Joined
    Sep 2007
    Posts
    60

    Prove Trig Identity

    \frac{cos2x}{sin2x+1} = \frac{1-tanx}{1+tanx}

    Can't seem to prove this one, help please
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Flow Master
    mr fantastic's Avatar
    Joined
    Dec 2007
    From
    Zeitgeist
    Posts
    16,948
    Thanks
    5
    Quote Originally Posted by Raj View Post
    \frac{cos2x}{sin2x+1} = \frac{1-tanx}{1+tanx}

    Can't seem to prove this one, help please
    Start by choosing a side.

    I choose the right hand side.

    Substitute \tan x = \frac{\sin x}{\cos x}. Multiply numerator and denominator by \cos x.

    Now multiply numerator and denominator by \cos x + \sin x and expand.

    You should recognise the numerator as \cos (2x). Simplify the denominator to recognise \sin (2x).
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Raj
    Raj is offline
    Junior Member
    Joined
    Sep 2007
    Posts
    60
    Substitute . Multiply numerator and denominator by .
    Where is the cosx coming from
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Flow Master
    mr fantastic's Avatar
    Joined
    Dec 2007
    From
    Zeitgeist
    Posts
    16,948
    Thanks
    5
    Quote Originally Posted by Raj View Post
    Where is the cosx coming from
    Did you do the substitution?

    If you did it should be perfectly clear where the cos x comes from. You multiply top and bottom by cos x to get rid of the fraction.

    You have to actually do each of the suggested steps before you'll see why you do them.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. prove the trig identity
    Posted in the Calculus Forum
    Replies: 1
    Last Post: June 9th 2010, 07:33 PM
  2. Prove given trig identity
    Posted in the Calculus Forum
    Replies: 8
    Last Post: May 3rd 2010, 07:04 PM
  3. Prove the following Trig Identity
    Posted in the Trigonometry Forum
    Replies: 4
    Last Post: April 2nd 2009, 08:56 AM
  4. Prove this trig identity.
    Posted in the Trigonometry Forum
    Replies: 6
    Last Post: November 26th 2008, 10:38 AM
  5. Prove the Trig Identity
    Posted in the Trigonometry Forum
    Replies: 7
    Last Post: February 24th 2008, 06:22 PM

Search Tags


/mathhelpforum @mathhelpforum