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Integrating Inverse Algebraic Equations

  • 13-02-2009 1:03pm
    #1
    Posts: 4,630 ✭✭✭


    Is there a more intuitive and logical way to integrate an inverse algebraic equation (i.e. 1/(x^2 + 4x + 29)) than factorising it and using an identity? The way it's done in school is to factorise the equation, and then use a "shortcut" from the tables (i.e. it becomes 1/a tan^-1 x/a). That way isn't hard to do at all, but, I'm wondering is there a more logical and intuitive way to do it, a way that you know what you're actually doing?

    I've been trying to do it using a substitution, differentiating the substitution twice (that way I get 1/2 du^2 = dx^2), then messing around a few ways using double integrals. But, I can't seem to get it.

    So is there a better way?

    Thanks.


Comments

  • Closed Accounts Posts: 773 ✭✭✭Cokehead Mother


    Okay I'm not really sure if this is the sort of thing you're looking for, but I'd use the same technique you use in LC to integrate root(a^2 - x^2).

    Your integral is int dx/[(x + 2)^2 + 5^2]

    5^2tan^2(u) + 5^2 = 5^2sec^2(u)

    and 5tan(u) differentiates to 5sec^2(u)

    So if you make the substitution x + 2 = 5tanu, it comes out nicely.


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