Results 1 to 4 of 4

Math Help - Trigo

  1. #1
    Junior Member
    Joined
    Sep 2008
    Posts
    59

    Trigo

    Thread can be closed, I have got the answer.
    Last edited by ose90; September 12th 2008 at 08:01 PM.
    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 ose90 View Post
    The function f is defined by f(x)=cos x + (1/2) cos 2x , 0 <= x <= 2pi

    find all values of x in the form of k pi, with k correct to one decimal places when f(x)=0

    My solution:

    cosx + 1/2 cos 2x = 0
    2 cos x + cos 2x = 0
    2 cos x + (2cos^2 x-1) = 0
    2 cos ^2 x + 2 cos x - 1 = 0

    Using quadratic formula, i get x = -1/2 +- (sqrt3)/2

    How should I express them in term of pi?
    You only need to solve \cos x = \frac{\sqrt{3} - 1}{2} (why?).

    Note that you only need them in the form k pi, with k correct to one decimal place. So use technology to get decimal approximations of all solutions and then solve for k, correct to one decimal place.

    Eg. One solution is x = 1.19606. So 1.19606 = k \pi => k = \, ....., correct to one decimal place.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Flow Master
    mr fantastic's Avatar
    Joined
    Dec 2007
    From
    Zeitgeist
    Posts
    16,948
    Thanks
    5
    Quote Originally Posted by ose90 View Post
    Thread can be closed, I have got the answer.
    Deleting your question and proclaiming that you have the answer is NOT good form.

    1. Other members might be interested in the question and its solution. MHF would be a pretty crummy resource if everyone did what you've done.

    2. The answer you've got might be wrong. Surely getting some sort of 'peer review' of your answer is desirable.

    3. Some members might have valuable contributions to make (like alternative methods of solution, interesting extensions to the original question etc.).

    There are other reasons but these are the three most important, in my opinion anyway.
    Last edited by mr fantastic; September 12th 2008 at 08:22 PM.
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Junior Member
    Joined
    Sep 2008
    Posts
    59
    Quote Originally Posted by mr fantastic View Post
    Deleting your question and proclaiming that you have the answer is NOT good form.

    1. Other members might be interested in the question and its solution. MHF would be a pretty crummy resource if everyone did what you've done.

    2. The answer you've got might be wrong. Surely getting some sort of 'peer review' of your answer is desirable.

    3. Some members might have valuable contributions to make (like alternative methods of solution, interesting extensions to the original question etc.).

    There are other reasons but these are the three most important, in my opinion anyway.
    alright and thanks for your advice, I will never repeat it
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Trigo Eqn
    Posted in the Trigonometry Forum
    Replies: 7
    Last Post: December 2nd 2009, 02:53 AM
  2. Trigo qn
    Posted in the Trigonometry Forum
    Replies: 4
    Last Post: October 8th 2008, 04:13 AM
  3. Trigo qn
    Posted in the Trigonometry Forum
    Replies: 2
    Last Post: October 7th 2008, 04:39 AM
  4. trigo sum/different~do not do~!
    Posted in the Trigonometry Forum
    Replies: 5
    Last Post: September 11th 2008, 06:43 AM
  5. trigo > i cant do it!
    Posted in the Trigonometry Forum
    Replies: 3
    Last Post: September 9th 2008, 05:43 AM

Search Tags


/mathhelpforum @mathhelpforum