How to set up partial fractions using linear factors?

In summary, To integrate the given function, it can be factored into linear factors and expressed as a series expansion. Alternatively, it can also be rewritten as a polynomial by subtracting out the singular terms, resulting in a solution without the need to solve equations.
  • #1
philadelphia
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Homework Statement



I need to intergrate the following

(4x2+2x-1)/ (x3+x2)
How to set up problem using linear factors?

The Attempt at a Solution



Factoring the denominator I get:
x2(x+1)

By linear/quadratic factoring I get:
A/x + Bx+C/ x2 + d/(x+1)

Is this right?
 
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  • #2
Correct. All you need to do is to solve for the coefficients.

If you don't like to do that, you can use the following alternative method. We have:

R(x) =(4x^2+2x-1)/ (x^3+x^2) =

(4x^2+2x-1)/ [x^2(1+x)]

Near x = -1 this is:

R(x) = 1/(1+x) * {[4x^2 + 2 x - 1]/x^2 at x = -1} =

1/(1+x)

Now, a more accurate expansion of R(x) near x = 1 would be obtained by multiplying 1/(1+x) by the expansion of [4x^2 + 2 x - 1]/x^2 around the point x = 1, then you would get an expansion of the form:

1/(1+x) [1 + A (1+x) + B (1+x)^2 + ...] =

1/(1+x) + A + B(1+x)^2


But only the first term in ths expansion is singual, it diverges as you let x tend to 1.

If we now consider the expansion of R(x) near the other singular point, x = 0, and collect all the singular terms, we get:

(4x^2+2x-1)/ [x^2(1+x)] =

1/x^2 [4x^2 + 2x - 1] series expansion of 1/(1+x) =


1/x^2 [4 x^2 + 2 x - 1] [1-x + x^2 - ...]=

-1/x^2 + 3/x + nonsingular terms.

So, we have that near x = 1, the singular behaviour of R(x) is given by 1/(1+x) while near x = 0 it is given by -1/x^2 + 3/x. Then it follows that the function:

R(x) - [1/(1+x) - 1/x^2 + 3/x]

is a rational function that has no singularities because the only singular points of R(x) are at zero and at x = 1 but we have subtracted the singular terms of R around these points. It thus follws that:

R(x) - [1/(1+x) - 1/x^2 + 3/x]

is a polynomial. But since both R(x) and the subtracted terms tend to zero at infinity this polynomial must be identical to zero. Therefore we have:

R(x) - [1/(1+x) - 1/x^2 + 3/x] = 0 ---------->

R(x) = 1/(1+x) - 1/x^2 +3/x

And we didn't need to solve a single equation to get this result.
 

FAQ: How to set up partial fractions using linear factors?

What is the purpose of setting up partial fractions?

The purpose of setting up partial fractions is to break down a complex fraction into smaller, simpler fractions. This can make solving equations and evaluating integrals easier and more manageable.

What are the steps to set up partial fractions?

The steps to set up partial fractions are as follows:
1. Factor the denominator of the given fraction.
2. Write the fraction as a sum of smaller fractions, with each denominator corresponding to one of the factors from the original denominator.
3. Set up equations with the numerators of the smaller fractions and solve for their values.
4. Combine the smaller fractions back into one fraction.

When is it necessary to use partial fractions?

Partial fractions are necessary when the denominator of a fraction cannot be easily factored, or when the fraction is part of a larger equation that needs to be solved. They are also commonly used in calculus when evaluating integrals involving rational functions.

What are some common mistakes to avoid when setting up partial fractions?

Some common mistakes to avoid when setting up partial fractions include:
- Forgetting to factor the denominator
- Incorrectly setting up equations for the numerators
- Not including all of the necessary fractions in the final combined fraction
- Forgetting to simplify the fractions before combining them back into one fraction.

Are there any shortcuts or tricks to setting up partial fractions?

There are some shortcuts or tricks that can be used when setting up partial fractions, such as:
- Using the "cover-up" method to determine the numerators
- Using the "Lagrange interpolation" method for more complicated fractions
- Using the "Heaviside cover-up" method for fractions with repeated factors in the denominator. However, it is important to fully understand the steps and reasoning behind setting up partial fractions before using these shortcuts.

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