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Math Help - Stuck on HW problem, can't see how I'm wrong

  1. #1
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    Stuck on HW problem, can't see how I'm wrong

    Hello all,

    The problem is this:

    =

    I worked it out like this:

    x^4/4 = 2b^4/4 - b^4/4

    But when I enter that in, they say I am wrong. Does anybody see what I'm doing wrong on this?
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  2. #2
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    earboth's Avatar
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    Quote Originally Posted by JohnJames View Post
    Hello all,

    The problem is this:

    =

    I worked it out like this:

    x^4/4 = 2b^4/4 - b^4/4

    But when I enter that in, they say I am wrong. Does anybody see what I'm doing wrong on this?
    You have to use brackets in the last line:

    \left[\dfrac{x^4}4\right]^{2b}_b = \dfrac{(2b)^4}4-\dfrac{b^4}4 = \dfrac{15b^4}4
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  3. #3
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    Quote Originally Posted by JohnJames View Post
    Hello all,

    The problem is this:

    =

    I worked it out like this:

    x^4/4 = 2b^4/4 - b^4/4

    But when I enter that in, they say I am wrong. Does anybody see what I'm doing wrong on this?
    \int_{b}^{2b}x^{3}dx=\left[ \frac{x^{4}}{4}+c\right]^{2b}_{b}=\frac{(2b)^{4}}{4}+c-(\frac{b^{4}}{4}+c)=4b^{4}-\frac{b^{4}}{4}=\frac{15b^{4}}{4}
    Last edited by coscos; July 13th 2010 at 09:50 AM. Reason: there you go
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  4. #4
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    Oh Wow, simple math got me this time lol, thanks to earboth and coscos
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