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Rolls Theorem (trig functions)

  1. Feb 23, 2015 #1
    1. The problem statement, all variables and given/known data
    f(x) = sin5x ; [π/5,2π/5] finding the point c which f'(x) =0. I understand the theorem and how to complete it, my issue is using the triq functions

    2. Relevant equations
    f'(x) = 5cos5x

    3. The attempt at a solution
    5cos5x=0
    cos5x=0
    5x=π/3
    x=π/15
    my answer is not correct, I am using mathlab and I cannot see how they came to their answer

    Edit: Mathlab answer is 3π/10
     
  2. jcsd
  3. Feb 23, 2015 #2
    cos5x=0
    5x=π/2 or 3π/2
    x=π/10 or 3π/10
    x=π/10 is not in domain so answer is 3π/10
     
  4. Feb 23, 2015 #3
    Thank you for the reply, I am obviously doing something wrong between;
    cos5x=0
    and
    5x= π/2 or 3π/2 ( i understand where you get 3π/2, just another revolution from π/2)
    what i don't understand is how you got π/2, I was under the impression that you take the inverse of cosine from each side of the equation to isolate x on the left hand side. Furthermore, I was told that finding the inverse of the cosine function is π/3, because that gives you 1/2. I think I was just given some back information on how to solve that portion of the problem.
     
  5. Feb 23, 2015 #4

    SteamKing

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    Science Advisor
    Homework Helper

    Not quite. Remember, 1 revolution = 2π radians, so it's a half revolution.

    If you were trying to find θ such that cos θ = 0, what would be the value of θ in this case? For what angles is the cosine zero?

    You can use the unit circle to figure this out, and you should have memorized where sin θ = 0, cos θ = 0, tan θ = 1, etc.
     
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