Results 1 to 4 of 4

Math Help - solve trigonometry equation

  1. #1
    Newbie
    Joined
    May 2008
    Posts
    3

    solve trigonometry equation

    Hey, I'm just like... completely zoned out:

    sin x = cos ( 2x + 15 )
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Forum Admin topsquark's Avatar
    Joined
    Jan 2006
    From
    Wellsville, NY
    Posts
    9,923
    Thanks
    332
    Awards
    1
    Quote Originally Posted by Kuczaj View Post
    Hey, I'm just like... completely zoned out:

    sin x = cos ( 2x + 15 )
    Recall that
    sin(x) = cos \left ( x + \frac{\pi}{2} \right )

    Thus
    sin(x) = cos \left ( x + \frac{\pi}{2} \right ) = cos(2x + 15)

    -Dan
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Newbie
    Joined
    May 2008
    Posts
    3
    Wow, thanks for that that, even you didn't solve for X. Anyways, I found the answer and it's 25.
    Follow Math Help Forum on Facebook and Google+

  4. #4
    MHF Contributor Reckoner's Avatar
    Joined
    May 2008
    From
    Baltimore, MD (USA)
    Posts
    1,024
    Thanks
    75
    Awards
    1

    Smile

    Quote Originally Posted by topsquark View Post
    Recall that
    sin(x) = cos \left ( x + \frac{\pi}{2} \right )

    Thus
    sin(x) = cos \left ( x + \frac{\pi}{2} \right ) = cos(2x + 15)

    -Dan
    This is incorrect. For example, let x = \frac{\pi}2. The actual identity should be \sin x = \cos\left(\frac{\pi}2 - x\right). Then we have:

    \cos\left(\frac{\pi}2 - x\right) = \cos(2x + 15)

    Note, however, that there will be infinitely many solutions due to the periodicity of the cosine.

    And Kuczaj, did you even try your answer? I get

    \begin{array}{rcr@{.}l}<br />
\sin 25^\circ & \approx & 0&42261826174\\<br />
\cos\left(2\left(25^\circ\right) + 15\right) & \approx & -0&986467363994<br />
\end{array}
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Trigonometry equation 2
    Posted in the Trigonometry Forum
    Replies: 3
    Last Post: December 8th 2009, 11:06 PM
  2. trigonometry; solve
    Posted in the Trigonometry Forum
    Replies: 5
    Last Post: August 31st 2009, 03:45 AM
  3. solve trigonometry equation
    Posted in the Trigonometry Forum
    Replies: 5
    Last Post: March 31st 2008, 06:05 AM
  4. Trigonometry to solve complex shapes
    Posted in the Trigonometry Forum
    Replies: 3
    Last Post: May 14th 2007, 02:28 PM
  5. Please help me (solve trigonometry equation)
    Posted in the Trigonometry Forum
    Replies: 3
    Last Post: November 26th 2006, 03:41 PM

Search Tags


/mathhelpforum @mathhelpforum