Results 1 to 3 of 3

Math Help - [SOLVED] Trig identities

  1. #1
    Junior Member
    Joined
    Jul 2008
    From
    Cyprus
    Posts
    45

    [SOLVED] Trig identities

    Given that cos x = e^jx + e^-jx /2 prove that 2cs^2(x)-1 =cos2x

    I know that the second part is double angle formula (not that that helps me prove the identitie) But i dont understand what the first part has to do with it. I'm fairly sure its demoivres. But that's the limit of what i understand.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Forum Admin topsquark's Avatar
    Joined
    Jan 2006
    From
    Wellsville, NY
    Posts
    9,962
    Thanks
    349
    Awards
    1
    Quote Originally Posted by ally79 View Post
    Given that cos x = e^jx + e^-jx /2 prove that 2cs^2(x)-1 =cos2x

    I know that the second part is double angle formula (not that that helps me prove the identitie) But i dont understand what the first part has to do with it. I'm fairly sure its demoivres. But that's the limit of what i understand.
    "2cs^2(x)-1 =cos2x"

    Huh?

    2 cos^2(x) - 1 = cos(2x)
    if that's what you mean.

    2 [(e^{jx} + e^{-jx})/2]^2 - 1

    = (1/2) (e^{2jx} + e^{-2jx} + 2) - 1

    Can you take it from here?

    -Dan
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Junior Member
    Joined
    Jul 2008
    From
    Cyprus
    Posts
    45
    yes thank you
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. [SOLVED] trig identities
    Posted in the Trigonometry Forum
    Replies: 4
    Last Post: November 17th 2009, 04:03 PM
  2. [SOLVED] Proving Trig Identities
    Posted in the Trigonometry Forum
    Replies: 4
    Last Post: May 10th 2009, 05:22 PM
  3. [SOLVED] Verifying Trig identities
    Posted in the Trigonometry Forum
    Replies: 1
    Last Post: February 5th 2009, 05:16 PM
  4. [SOLVED] Trig Identities
    Posted in the Trigonometry Forum
    Replies: 1
    Last Post: February 1st 2009, 09:29 AM
  5. [SOLVED] Proving Trig Identities
    Posted in the Trigonometry Forum
    Replies: 3
    Last Post: November 13th 2008, 04:45 PM

Search Tags


/mathhelpforum @mathhelpforum