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

Difficulty solving the following question

  1. Sep 23, 2006 #1
    Hi, having difficulty solving the following question.. Any1 can help me and guide me through solving it ?? or is there any good website or reference to recommand ?? Thankz alot..

    Attached Files:

    • Q1.JPG
      File size:
      6.4 KB
  2. jcsd
  3. Sep 24, 2006 #2
    The integral is undefined. However, if you were to define it, there's a particular value that seems pretty natural. To help you guess what that might be, notice that [itex]x(t)[/itex] is an odd function:

    [tex]x(-t) = (-t)^3\cos (-10t)^3 = -t^3\cos (10t)^3 = -x(t).[/tex]

    Does that help? :smile:

    (the "natural" value for the integral that I'm speaking of is the Cauchy principal value)
  4. Sep 24, 2006 #3
    Oh.. I only know the shortcut method but don't know whether u are refering to it or not...

    I only know that 't' is odd function and cos(10t) is even function.. so multiply odd with even will give me odd function...

    I'm confused is the part on how to solve it mathematically said integration by parts.. haha ...
  5. Sep 24, 2006 #4
    To integrate that function integration by parts would work, but would probably be extremely messy, the problem you will run into is evaluating the integral since both limits will be infinite and the improper integral will be undefined as Data said above.
  6. Sep 24, 2006 #5
    If you wanted to use integrate by parts you could probably set the upper and lower limits of integration to some constant [itex]\pm a[/itex]. This symmetric choice of limits will allow you to evaluate the integral by parts, by utilising the symmerty properties of the the now-integrated function to determine what cancellations occur etc. Then in the final step take the limit of [itex]|a|\to\infty[/itex].
  7. Sep 24, 2006 #6
    what about contour integration?
  8. Sep 24, 2006 #7
    That is precisely taking the Cauchy principal value. It is trivial to see what the result will be, because x is odd.
  9. Sep 24, 2006 #8
    Okie.. Thank every1 for the guide and comment given.
  10. Sep 25, 2006 #9
    shouldn't the result be zero since the function is odd?
  11. Sep 25, 2006 #10


    User Avatar
    Science Advisor

    Yes, for any A,
    [tex]\int_{-A}^{A} t^3 cos^3(10t)dt= 0[/tex]
    so the limit as A->infinity, the "Cauchy Principal Value" would be 0. That's what everyone has been saying.

    Data said that integral is undefined because the actual improper integral must be
    [tex]\int_{-A}^B x^3 cos^3(10t)dt[/tex]
    with A and B going to infinity independently. That is undefined.
  12. Sep 25, 2006 #11
    tat true.. if the limit is from A to -A or even from Infinity to -Infinity, the result will be zero. If it is undefined, say like limit from A to B den the answer should not be zero. right ?

    This question in the first place was just to solve it to be odd function using mathematical approach. kekez..

    Did I said anything wrong ??? If yes, pls correct mi. thankz.. heez.
  13. Sep 25, 2006 #12


    User Avatar
    Science Advisor

    Yes, the "even from infinity to -infinity" is wrong. As I said before
    [tex]\int_{-\inty}^\infty f(x) dx= \lim_{A\rightarrow -\infty}\lim_{B\rightarrow \infty} \int_A^B f(x)dx[/tex]

    Certainly, if an integral is "undefined", then it is not 0! The original question was to find the value of that integral if it existed. I don't know what you mean by "solve it to be odd function".
  14. Sep 26, 2006 #13
    From observation, we already know that the function is an odd function, but What i mean is solve and proof it that the equation is an odd function. what i mean is solve and proof it mathematically if possible.
  15. Sep 26, 2006 #14


    User Avatar
    Science Advisor

    I have absolutely no idea what you mean. You say "What i mean is solve and proof it that the equation is an odd function" but you had just said "we already know that the function is an odd function". Prove what mathematically?
    That the integral given doesn't exist?

    That the Cauchy Principal Value is 0?
  16. Sep 26, 2006 #15
    ok.. thank.. maybe i should read up cachy principal again..
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook