Partial Fractions

**ubhik** · Apr 28th 2008, 04:28 AM

I am in the middle of solving a integro-differential, and I need to use inverse Laplace transforms to complete the last bit, I have the following:

$ x(s)= \frac{1}{s\left[1+\frac{s}{s+1}\right]}+ \frac{1}{s^2\left[1+\frac{s}{s+1}\right]}-\frac{1}{s+1\left[1+\frac{s}{s+1}\right]} $

I have simplified this to the following form:

$ x(s)= \frac{s+1}{s(s-2)}+\frac{s+1}{s^2(2s+1)}-\frac{s+1}{(s+1)^2+s} $

Then using partial fractions I have taken the above term by term, and done the following:

$ \frac{s+1}{s(s-2)} = \frac{-1/2}{s}+\frac{3/2}{s-2} $

Second term:

$ \frac{s+1}{s^2(2s+1)} = \frac{-1}{s}+\frac{1}{s^2}+\frac{2}{2s+1} $

Can some please varify if I have used partial fractions correctly on the first 2 expressions

Also I do not understand how to use partial fraction on the third expression, can some please help and explain how I would do this... ?

**mr fantastic** · Apr 28th 2008, 04:42 AM

Originally Posted by ubhik

I am in the middle of solving a integro-differential, and I need to use inverse Laplace transforms to complete the last bit, I have the following:

$ x(s)= \frac{1}{s\left[1+\frac{s}{s+1}\right]}+ \frac{1}{s^2\left[1+\frac{s}{s+1}\right]}-\frac{1}{s+1\left[1+\frac{s}{s+1}\right]} $

I have simplified this to the following form:

$ x(s)= \frac{s+1}{s(s-2)}+\frac{s+1}{s^2(2s+1)}-\frac{s+1}{(s+1)^2+s} $

Then using partial fractions I have taken the above term by term, and done the following:

$ \frac{s+1}{s(s-2)} = \frac{-1/2}{s}+\frac{3/2}{s-2} $

Second term:

$ \frac{s+1}{s^2(2s+1)} = \frac{-1}{s}+\frac{1}{s^2}+\frac{2}{2s+1} $

Can some please varify if I have used partial fractions correctly on the first 2 expressions

Also I do not understand how to use partial fraction on the third expression, can some please help and explain how I would do this... ?

You have.

(s+1)^2+s to get s^2 + 3s + 1. This factorises, but not nicely.

**Isomorphism** · Apr 28th 2008, 06:25 AM

Originally Posted by ubhik

$ x(s)= \frac{s+1}{s(s-2)}+\frac{s+1}{s^2(2s+1)}-\frac{s+1}{(s+1)^2+s} $

Also I do not understand how to use partial fraction on the third expression, can some please help and explain how I would do this... ?

Break it in a different way:

$\frac{s+1}{s^2+3s+1} = \frac{s+1}{(s+\frac32)^2 - \frac54}$

Thread: Partial Fractions

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