Integrating a Rational Function with a Quadratic Denominator

In summary, the student is trying to solve a homework equation involving partial fractions, but is having trouble understanding why substitutions are being made. He eventually solves the equation using a different method after finding C and D.
  • #1
n4rush0
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Homework Statement


[1/(x^4 - x)]dx


Homework Equations





The Attempt at a Solution


I factored the denominator to x(x-1)(x^2 + x +1) and I'm not sure if I can use partial fractions.
 
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  • #2
Welcome to PF n4rush0.

What exactly is preventing you from carrying out partial fractions? That is the correct way to proceed.
 
  • #3
So I set

1/[y(y-1)(y^2+y+1)] = A/y + B/(y-1) + (Cy+D)/(y^2+y+1)

1 = (y-1)(y^2+y+1)A + y(y^2+y+1)B + y(y-1)(Cy+D)

y = 0, A = -1
y = 1, B = 1/3

Not sure what to do for C and D
 
  • #4
You're doing something wrong. Just write the whole polynomial in y and equal it to 1. You should get a system of 4 equations with 4 unknowns which is very easy to solve.

EDIT: You're doing it right, but it's a much more tedious work towards the end than in the method I suggested.
 
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  • #5
bigubau said:
You're doing something wrong. Just write the whole polynomial in y and equal it to 1. You should get a system of 4 equations with 4 unknowns which is very easy to solve.

No he's doing it right, it's just a different method. However...

n4rush0: Do you notice the pattern of your substitutions? It's to get some numbers in front of just 1 term, and zero's in front of the rest, right? To find C and D, you need to substitute in the zeros of y^2+y+1. You'll end up with a pair of simultaneous equations for C and D, which you can solve.

However, in this case, since you already know A and B, it's easier at that point of the problem to just sub in simple values for y, say -1 and 2, and solve those.

Alternatively, you can do what bigubau suggested.
 
  • #6
n4rush0 said:
So I set

1/[y(y-1)(y^2+y+1)] = A/y + B/(y-1) + (Cy+D)/(y^2+y+1)

1 = (y-1)(y^2+y+1)A + y(y^2+y+1)B + y(y-1)(Cy+D)

y = 0, A = -1
y = 1, B = 1/3

Not sure what to do for C and D
Out of curiosity, why did you switch from x that was used in your integral to y here?
 
  • #7
I tried multiplying it out and got
1 = y^3 (A+B+C) + y^2 (B-C+D) + y(B-D) - A

Even knowing that A = -1 and B = 1/3, I'm still not seeing the "nice" solution.

Oh and, my original problem involved using y, but I wanted to use Mathmatica which solves dx integrals and I copied and pasted [1/(x^4 - x)]dx from there.
 
  • #8
Well, then you've missed the most important point in the partial fraction decomposition method.

on the LHS of what you wrote there's supposed to be the following polynomial in y

0 y^3 + 0 y^2 + 0y +1.

Does that help you solve the problem ?
 
  • #9
Yes, that helps alot. Thanks, I solved it.
 

FAQ: Integrating a Rational Function with a Quadratic Denominator

What is the general formula for integrating 1/(x^4 - x)?

The general formula for integrating 1/(x^4 - x) is ∫1/(x^4 - x)dx = 1/3ln|x^3 - 1| + C.

What is the substitution method for solving the integral of 1/(x^4 - x)?

The substitution method for solving the integral of 1/(x^4 - x) involves substituting u = x^2 - 1, du = 2xdx, and rewriting the integral as ∫1/(u^2 + 1)du.

How do you solve the integral of 1/(x^4 - x) using partial fractions?

To solve the integral of 1/(x^4 - x) using partial fractions, we first use long division to rewrite the integrand as 1/(x^4 - x) = 1/(x(x^3 - 1)) = 1/(x(x-1)(x^2 + x + 1)). Then, we can use the method of partial fractions to decompose the rational function into simpler fractions, and integrate each part separately.

Can the integral of 1/(x^4 - x) be solved using trigonometric substitution?

Yes, the integral of 1/(x^4 - x) can be solved using trigonometric substitution. We can use the substitution x = tanθ to rewrite the integral as ∫1/(tanθ(tan^3θ - tanθ))sec^2θdθ, and then use the trigonometric identity tan^2θ = sec^2θ - 1 to simplify the integrand.

Are there any other methods for solving the integral of 1/(x^4 - x)?

Yes, there are other methods for solving the integral of 1/(x^4 - x), such as using the Weierstrass substitution or the Euler substitution. However, these methods may not always be straightforward and may require a deeper understanding of advanced calculus concepts.

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