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Math Help - [SOLVED] substitution indefinite integrals

  1. #1
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    [SOLVED] substitution indefinite integrals

    Hi
    Could someone please explain this substitution problem? I keep getting the wrong answer :s

    ∫ [(9x) / ((x^4)+1)] dx

    Thank-you!
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  2. #2
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    \int\frac{9x}{x^{4}+1}dx

    Let u=x^{2}, \;\ \frac{du}{2}=dx

    This gives:

    \frac{9}{2}\int\frac{1}{u^{2}+1}du

    Now, it's easy.
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