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Math Help - Problem with Integration: U-substitution

  1. #1
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    Problem with Integration: U-substitution

    Hey guys, I'm having trouble understanding an aspect of u-substitution. In this problem:
    Integrate: x^3/(5*x^4+2)^3 * dx

    u = 5*x^4 + 2
    1/20 * du = x^3 *dx

    So that takes care of the x^3 * dx in the first equation, and the rest of the integration is easy.


    But I don't understand why the same principle can't be applied here:
    Integrate: x * sqrt(x-3) * dx
    u = x-3
    du = x * dx

    Why can't you cancel out x * dx with du here?

    Thanks in advance for any help.
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  2. #2
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    Quote Originally Posted by bobbert116 View Post
    u = x-3
    du = x * dx
    du = dx not x dx ..
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