Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Trigonometry question

  1. Feb 27, 2012 #1
    Hello,

    I'm studying for an exam and am stuck on a trigonometry question. Could anybody help me out?

    The question is:

    Solve the equation cos3x = 1/2, for x is an element of R, (where x is in radians)

    The answer is:

    x = pi/9 + 2npi/3

    or

    x = 5pi/9 + 2npi/3

    where n is an element of Z

    Sincere thanks

    John

    My attempt

    Well we know that cos (pi/3) = 1/2

    But cos 3x = 1/2 and are trying to figure out what x is.

    So what times 3 will give you pi/3. That would be pi/6.

    3 * (pi/6) = pi/3

    cos (3*(pi/6)) = 1/2
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Feb 27, 2012 #2
    If
    'Well we know that cos (pi/3) = 1/2'
    Then
    cos 3x = 1/2
    implies
    3x=pi/3
    implies
    x=pi/9
     
  4. Feb 27, 2012 #3
    Thanks,

    It says that the answer is


    x = pi/9 + 2npi/3

    or

    x = 5pi/9 + 2npi/3
     
  5. Feb 27, 2012 #4
    That is correct, when is Cos(a)=Cos(b)?
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook