# Math Help - residues

1. ## residues

Hi,

I'm working through an example of using residues to evaluate an improper integral, but I don't see how they arrive at the following initial step:

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

Any help would be appreciated

Thanks

2. $\frac{1}{x^2+1} = \frac{1}{(x+i)(x-i)}=\frac{a}{x+i}+\frac{b}{x-i}$

3. Originally Posted by ihr02
Hi,

I'm working through an example of using residues to evaluate an improper integral, but I don't see how they arrive at the following initial step:

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

Any help would be appreciated

Thanks
Please post the entire question. Thankyou.

4. Sorry about that. The question is using residues to evaluate the integral from 0 to infinity:

(x^2 / (x^2 + 1)(x^2 + 4))

Thanks

5. Originally Posted by ihr02
Sorry about that. The question is using residues to evaluate the integral from 0 to infinity:

(x^2 / (x^2 + 1)(x^2 + 4))

Thanks
This function has simple poles at values of x such that x^2 + 1 = 0 and x^2 + 4 = 0 ....

6. Originally Posted by ihr02
Hi,

I'm working through an example of using residues to evaluate an improper integral, but I don't see how they arrive at the following initial step:

x^2 / ((x^2 + 1)(x^2 +4)) = 1/3[ (4 / (x^2 + 1) - (1 / (x^2 + 4)]
That initial step just consists of using partial fractions to write $\frac{x^2}{(x^2+1)(x^2+4)} = \frac A{x^2+1} + \frac B{x^2+4}$, for suitable constants A and B. (If that looks odd, replace $x^2$ by a single variable, say $x^2=t$, to make it look more like a standard partial fractions decomposition.)

Why the worked example ahould have started in this way is a bit of a mystery. As Mr F points out, the way to solve the problem is to use the residue theorem, and it doesn't seem necessary to use partial fractions in order to find the residues.