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Homework Help: Periodic function problem

  1. Apr 28, 2006 #1

    Aya

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    I can someone pleas help me with this problem? Thanks.

    A periodic function f has a period of 12. If f(7)=-2 and f(11)=9, determine the value of f (43)

    how do you do this?
     
  2. jcsd
  3. Apr 28, 2006 #2
    Well, 43 - 7 = 36.
     
  4. Apr 28, 2006 #3

    dav2008

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    Gold Member

    If f(x) has a period of a that means f(x)=f(x+na) where n is an integer.

    Can you figure out the problem using that fact?
     
  5. Apr 30, 2006 #4

    Aya

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    ^ I don't get it can u show me an example?
    or would it be like this

    f(x)=f(x+na)
    f (43)=f(7+12(-2))
    f(43)=-17
     
  6. Apr 30, 2006 #5

    dav2008

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    It just means that if the period is 12 then f(43)=f(43+n*12) where n is an integer.

    So f(43)=f(43+12)=f(43-12)=f(43+24)...

    In your example you say that f(43)=f(7+12(-2)). This is true but it means that f(43)=f(-17).

    You are given f(7) and f(11). Can you relate one of those to f(43) somehow?
     
  7. Apr 30, 2006 #6

    Aya

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    f(7)=-2

    f(7)=(7+12+12+12)
    =f(43)

    so f(43)=-2

    is that right, what about f(11)? or do i not need to use that?
     
  8. Apr 30, 2006 #7

    dav2008

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    Yes that's right.

    You found out f(43) so I don't know why you're worried about f(11).
     
  9. Apr 30, 2006 #8

    Aya

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    ^ ok, thanks
     
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