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!

I need help

  1. Aug 30, 2005 #1
    Suppose that the function T has period 7 and that T(0)=3.

    Explain why T(-30)=T(40)?

    Is it normal that teacher just tells you to read stuff in book and then do your homework, without explaining it? She was "teaching" some stupid stuff about graphing f(x)=(x-1)^2 + 2, which they teach you in Alg I and then gives you questions about periodic equations. Last two days one girl and me were the only ones to be able to do the homework. :mad:
  2. jcsd
  3. Aug 30, 2005 #2


    User Avatar
    Gold Member

    If a function has a period, that means it repeats. Since the period is seven, that means that T(0)=T(7)=T(14)=... and that T(1)=T(8)=T(15)=.... In general it means that whatever value the function has at "a", it will have the same value at "a+7", "a+14", and so on. Do you see the solution now?
  4. Aug 30, 2005 #3

    Doc Al

    User Avatar

    Staff: Mentor

    What's the meaning of period? If the period is 7 (as given), what can you say about T(x) compared to T(x +7)?
  5. Aug 30, 2005 #4
    I understand that every 7 the function repeats. But if I go 7 in positive and 7 in negative direction then I should have T(40)=T(-40) or T(30)=T(-30). Or am I missing something?
  6. Aug 30, 2005 #5
    O damn. I got it. My bad. Thanks for help.
  7. Aug 30, 2005 #6

    Doc Al

    User Avatar

    Staff: Mentor

    You're missing something. Complete this list:
    T(-30) = T(-30 + 7) [which is T(-23)] = T(-23 +7) ... and so on...
  8. Aug 30, 2005 #7
    Shame on me. I calculated that there are 50 numbers between -30 and 40. I guess I'm to pised off.
  9. Aug 30, 2005 #8


    User Avatar
    Science Advisor

    Last I checked 40-(-30)= 70 which is a pretty obvious multiple of 7!
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook