1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Trigonometric limit question

  1. Mar 20, 2009 #1
    1. The problem statement, all variables and given/known data
    Let f(x)=cos (x). Denote f^2(x) = ff(x), f^3(x)=fff(x) and so on.
    For x=100 radians, find f^n(x) for n=1 to 20
    Develop a calculator technique to find f^n(x) for any given value of n. Hence, find correct to 7 decimal places, the limit of f^n(x) for x= 100 radians and n approaching infinity.

    2. Relevant equations

    3. The attempt at a solution
    sorry,i have no idea how to tackle this question.can any1 help me?=)
    Last edited: Mar 20, 2009
  2. jcsd
  3. Mar 20, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper

    Any vague ideas? The problem just asks for a 'calculator technique'. I would just set my calculator for radian mode, put in 100 and then just keep pushing 'cos', 'cos', 'cos'...
  4. Mar 20, 2009 #3
    oh yeah...i tot we need to develop another different formula or method to calculate the value rather than using the calculator and cos cos cos.
  5. Mar 21, 2009 #4


    User Avatar
    Homework Helper

    hi lightningz

    so use the caclulator to see what happens with repeated applications, then see if you can work out why...

    conceptually, you could imgine the process as taking:
    x1 point = 100radians, y1 = cos(x 1)

    then transfer that back to the x axis using the line y = x, so x2 = y1 = cosx, now repeat the process
    y2 = cos(x 2)= cos(cos(x 1))

    try drawing it out for a few cycles (after the first it will be near the y axis)
    is there any pattern you can see? can you describe it?
  6. Mar 21, 2009 #5
    the graph become smaller and smaller(ie converging to a certain value) and when n=infinty, it actually becomes a straight line. in this case, limit of cos^n(x),(x--->infinity), for all value of x is that particular value..that's what i got...urm..is that what the question want?=)
  7. Mar 21, 2009 #6


    User Avatar
    Homework Helper

    which graph?

    i assume you are plotting:
    [tex] f^n(100) = Cos(f^{n-1}(100)) [/tex] against n

    did you try what i mentioned? plot the line f(x) = cos(x) and g(x) = x and trace the operation of the function by moving a point between those 2 lines, this should illuminate what is going on

    based on your question its probably enough to hit cos on your calculator enough times that the numbers down to the seventh decimal stop changing... probably about 40 or so times (easier in a speadhseet)

    though there is a bit more going on here - have you heard of a fixed point of a function before?
  8. Mar 21, 2009 #7
    sorry.i never heard of fixed point.
    well the intersection point of the two graphs f(x)= cos (x) and g(x)=x..i got it by plotting in my calculator..which is the value for limit of cos^n(x),(x--->infinity), is that point called fixed point? thanks a lot for helping all the time =)
  9. Mar 21, 2009 #8


    User Avatar
    Homework Helper

    fixed point of a function where f(x) = x, so repeated application of the function does not cahneg the value at teh fixed point

    have a look here, it actually has a picture of f(x) = cos(x) as I was trying to explain
    http://en.wikipedia.org/wiki/Fixed_point_(mathematics [Broken])
    Last edited by a moderator: May 4, 2017
  10. Mar 21, 2009 #9
    thanks.i think i understood already =) such a nice guy you are..:wink:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook