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Math Help - Evaluate integral

  1. #1
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    Evaluate integral

    Integral (x^2-x+8)/(x^3+2x) dx

    I believe we have to do partial fraction decomposition to start this, but I'm not sure which partial fraction decomposition rule this would fall under.

    Any help in the right direction would be appreciated!
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  2. #2
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    Re: Evaluate integral

    a/x+(bx+c)/(x^2+2)
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    Re: Evaluate integral

    Thanks a ton!
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    Re: Evaluate integral

    Since the degree of the numerator is less than that of the denominator, you do not need to do a polynomial division, so you can proceed to the next step, which is to factor the denominator and set up the partial fraction decomposition:

    x^3+2x = x(x^2+2), and x^2+2 can not be factored, so write:

    \frac{x^2-x+8}{x^3+2x}=\frac{A}{x}+\frac{Bx+C}{x^2+2}

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    Re: Evaluate integral

    Thanks! So would the next step be multiplying by the common denominator giving:
    x^2-x+8 = A(x^2 +2) + x(Bx+C) ?
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  6. #6
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    Re: Evaluate integral

    That is correct. What do you think you should do next?
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    Re: Evaluate integral

    Quote Originally Posted by Prove It View Post
    That is correct. What do you think you should do next?
    Thanks! I have to find out what A,B,C equals. So I'll do when x=0,1,2 So A=4 when x=0. I dont understand this part: why does 1=A+B and B=-1 and therefore C=-1?

    I just went on to solve and got the integral of 4/x + -3x-1/x^2+2
    which simplifies to to integral (4/x -3 x/x^2+2 -1/x^2+2) dx

    that equals 4ln|x|-(3/2)ln(x^2+2)-(1/sqrt2)tan^-1(x/sqrt2)

    the bolded parts I would really appreciate any sort of simple explanation (like teaching it to a 5 yr old hahaha). thanks!
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  8. #8
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    Re: Evaluate integral

    What I would do from  \displaystyle x^2 - x + 8 = A\left( x^2 + 2 \right) + x \left( B\,x + C \right) would be to expand all the brackets on the right, simplify, then equate like powers of x.

    \displaystyle \begin{align*} x^2 - x + 8 &= A\, x^2 + 2A + B\,x^2 + C\,x \\ 1x^2 + \left( -1 \right) x + 8 &= \left( A + B \right) x^2 + C\, x + 2A \\ A + B = 1 \textrm{ and } C = -1 \textrm{ and } 2A &= 8 \\ A = 4 \textrm{ and } B = -3 \textrm{ and } C &= -1 \end{align*}

    So that means

    \displaystyle \begin{align*} \int{\frac{x^2 - x + 8}{x^3 + 2x}\,dx} &= \int{ \frac{4}{x} + \frac{-3x - 1}{x^2 + 2} \,dx } \\ &= \int{\frac{4}{x}\,dx} - \int{\frac{3x}{x^2 + 2} \,dx} - \int{ \frac{1}{x^2 + 2} \,dx } \\ &= \int{\frac{4}{x}\,dx} - \frac{3}{2}\int{\frac{2x}{x^2 + 2}\,dx} - \int{ \frac{1}{x^2 + 2}\,dx } \end{align*}

    Now the first integral is a logarithm, the second is solved using a substitution \displaystyle u = x^2 + 2 \implies du = 2x\,dx and the third is solved using a substitution \displaystyle x = \sqrt{2}\tan{(\theta)} \implies dx = \sqrt{2}\,sec^2{(\theta)}\,d\theta.
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