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Math Help - Why does this integral equal zero

  1. #1
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    Why does this integral equal zero



    Why does it equal to zero? I know it has something to do with sine being an odd function, but why does the integral of an odd function equal 0 ? (if it does)
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  2. #2
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    Quote Originally Posted by kamykazee View Post


    Why does it equal to zero? I know it has something to do with sine being an odd function, but why does the integral of an odd function equal 0 ? (if it does)
    If it exists it must be zero as the integrand is an odd function.

    This is because if the integral of odd function f(x) exits it is:

    \displaystyle I=\lim_{L\to \infty} \int_{-L}^L f(t)\ dt = \lim_{L\to \infty} \left[\int_{-L}^0 f(t)\ dt + \int_{0}^L f(t)\ dt  \right]

    .... \displaystyle =\lim_{L\to \infty} \left[\int_{0}^L f(-t)\ dt + \int_{0}^L f(t)\ dt  \right]=\lim_{L\to \infty} \left[\int_{0}^L -f(t)\ dt + \int_{0}^L f(t)\ dt  \right]=0

    Alternatively draw a picture and it will be obvious.

    CB
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  3. #3
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    When you have e negative integral from minus infinity to 0, the minus sign in front of the integral changes it to being from 0 to minus infinity, correct? Then how come it appears as being from 0 to positive infinity? Does that sign change aswell?

    Also, i don't think i know how to draw the graph of that function.

    And, how did you write the reply using math symbols (integral symbol, limit with subscript, etc) directly onto the reply?
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  4. #4
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    Quote Originally Posted by kamykazee View Post
    When you have e negative integral from minus infinity to 0, the minus sign in front of the integral changes it to being from 0 to minus infinity, correct? Then how come it appears as being from 0 to positive infinity? Does that sign change aswell?
    It is not at all clear what you are asking here! If f(x) is an odd function, then f(-x)= -f(x). I think that, by "the minus sing in front of the integral" you are referring to the minus sign you get from f(-x)= -f(x). But to get that, you have to change from "f(x)" to "f(-x)".

    \int_{-a}^a f(x)dx= \int_{-a}^0 f(x)dx+ \int_0^a f(x) dx.

    Now, make the change of variable u= -x in the first integral. Since u= -x, du= -dx and the lower limit is -(-a)= a:
    \int_{-a}^a f(x)dx= -\int_a^0 f(-u)du= \int_0^a f(-u)du= -\int_0^a f(u)du.

    If f(x) has anti-derivative F(x) (remember Captain Black said "if it exists ...") then -\int_0^a f(u)du= -F(a)+ F(0).

    Of course \int_0^a f(x)dx= F(a)- F(0) so the two parts, \int_{-a}^0 f(x)dx and \int_0^a f(x)dx cancel.


    Also, i don't think i know how to draw the graph of that function.
    As far as drawing the graph is concerned, I don't think Captain Black intended for you to draw a detailed graph (although that wouldn't be difficult using a graphing calculator). But you should be able to see that since f(-x)= -f(x), any region where f(x)> 0 to the right of x= 0 is mirrored by a region to the left where f(x)< 0 and those areas cancel each other.


    And, how did you write the reply using math symbols (integral symbol, limit with subscript, etc) directly onto the reply?
    Go back to the home page and look under "Math Resources" "Latex Help".
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    Nevermind, i just realised what i said was completely off.

    Thanks for the LaTex info
    Last edited by kamykazee; November 24th 2010 at 04:09 AM.
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    Well you said that -\int_0^a f(u)du= -F(a)+ F(0).

    and that \int_0^a f(x)dx= F(a)- F(0)

    But isn't F(a) from the first integral different from the F(a) in the second, since u is different than x, wouldnt that make the two F(a)'s to be different, thus not cancelling? (u=-x, which means its different than x, correct?) Basicly what im saying is, the function f(u) is different from f(x), since u is not equal to x, which would mean their integrations would be different aswell, wouldn't it?
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  7. #7
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    If f is any odd integrable function whatsoever then \displaystyle\int_{ - L}^L {f(x)dx}  = 0.
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    Well so i heard but, that doesn't explain my above post, i think im missing something.
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  9. #9
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    Quote Originally Posted by kamykazee View Post
    Well you said that -\int_0^a f(u)du= -F(a)+ F(0).

    and that \int_0^a f(x)dx= F(a)- F(0)

    But isn't F(a) from the first integral different from the F(a) in the second, since u is different than x, wouldnt that make the two F(a)'s to be different, thus not cancelling? (u=-x, which means its different than x, correct?) Basicly what im saying is, the function f(u) is different from f(x), since u is not equal to x, which would mean their integrations would be different aswell, wouldn't it?
    u and x are dummy variables of integration, they are arbitary names and can be changed at will, what is inportant is tah the 0's and a are the same.

    CB
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  10. #10
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    Quote Originally Posted by kamykazee View Post
    Well you said that -\int_0^a f(u)du= -F(a)+ F(0).

    and that \int_0^a f(x)dx= F(a)- F(0)

    But isn't F(a) from the first integral different from the F(a) in the second, since u is different than x, wouldnt that make the two F(a)'s to be different, thus not cancelling?
    No, it isn't. \int_a^b f(x)dx= \int_a^b f(u)du= \int_a^b f(t)dt= \int_a^b f(y)dy= \cdot\cdot\cdot
    The variable inside the integand is a "dummy" variable. It does not matter what you call it.

    (u=-x, which means its different than x, correct?) Basicly what im saying is, the function f(u) is different from f(x)
    No, the functions in f(x), f(u), f(t), f(y), etc. are all the same function- f.

    , since u is not equal to x, which would mean their integrations would be different aswell, wouldn't it?
    No, you need to review your "function notation".
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  11. #11
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    Thank you all for your replies and clarifying the problem.
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