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Thread: I need help with definite integrals.

  1. #1
    Junior Member
    Oct 2010

    I need help with definite integrals.

    In, class when we were attempting questions in which we used to find area bounded by curves through definite integrals, we used to look for the interval in which the function would be negative, then we'd Divide the whole GIVEN INTERVAL into Sub-intervals in which the function was either positive OR negative. We then used to calculate the areas in these subintervals and from here the confusing part starts...

    We put a negative sign with the areas under the x-axis, and then added them to the rest of the areas...fair enough and we understood what it meant....

    HOWEVER, when we were solving simple definite integrals (where we were not asked about area) we did not care abt the values for which the function was positive...Instead, we just solved it by putting in the limits of the integral and we got the answer......

    Im now abit confused?...Why didnt we care for the "negative values of the function" in solving SIMPLE DEFINITE INTEGRALS, just like we did in solving INTEGRALS FOR AREAS?

    By the way....Nevermind.......
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  2. #2
    MHF Contributor
    Prove It's Avatar
    Aug 2008
    A definite integral may or may not represent area. It depends on the context of the question.

    If you're asked to evaluate the area, then you will need to add all the absolute values of the definite integrals (which is equivalent to subtracting the negative definite integrals) because there is no such thing as negative area.

    However, if you are simply asked to find a definite integral, it may be for some other purpose, so you would just evaluate the definite integral as is.
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