Partial Fractions Expansion

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  • #1
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Homework Statement



Find the partial fractions expansion in the following form,

[tex]G(s) = \frac{1}{(s+1)(s^{2}+4)} = \frac{A}{s+1} + \frac{B}{s+j2} + \frac{B^{*}}{s-j2}[/tex]

Homework Equations





The Attempt at a Solution



I expanded things out and found the following,

[tex]1 = A(s^{2} + 4) + B(s^{2} + (1-j2)s -j2) + B^{*}(s^{2} + (1+j2)s + j2)[/tex]

From this I get the following equations,

[tex]A + B + B^{*} = 0[/tex]

[tex]B(1-j2) + B^{*}(1+j2) = 0[/tex]

[tex]4A - Bj2 + B^{*}j2 = 1[/tex]

This doesn't seem like a pleasant set of equations to solve.

I did another partial fractions expansion like so,


[tex]G(s) = \frac{1}{(s+1)(s^{2}+4)} = \frac{D}{s+1} + \frac{Es + F}{s^{2}+4}[/tex]

and found,

[tex]D = \frac{1}{5}, E = - \frac{1}{5}, F = \frac{1}{5}[/tex]

but this doesn't give me the form I need.

Any ideas? Any convenient ways to solve this?
 

Answers and Replies

  • #2
jbunniii
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Hint: if ##B^*## is the complex conjugate of ##B##, then ##B+B^* = 2\text{Re}(B)## and ##B - B^* = 2i\text{Im}(B)##. You can use these facts to simplify the unpleasant equations you obtained.
 
  • #3
lurflurf
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solve by inspection

$$\frac{1}{(s+1)(s^{2}+4)} =\frac{1}{(s+1)((-1)^{2}+4)}+\frac{1}{(2j+1)(2j+2j)(s-2j)}$$
$$+\frac{1}{(-2j+1)(s+2j)(-2j-2j)}= \frac{A}{s+1} + \frac{B}{s+2j} + \frac{B^{*}}{s-2j}$$

in general

$$\prod_k \frac{1}{x-a_k}=\sum_l \frac{1}{x-a_l}\prod_{k \ne l} \frac{1}{a_l-a_k}$$
 
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  • #4
SammyS
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Hint: if ##B^*## is the complex conjugate of ##B##, then ##B+B^* = 2\text{Re}(B)## and ##B - B^* = 2i\text{Im}(B)##. You can use these facts to simplify the unpleasant equations you obtained.
or in Engineering terms where usually ##\ j=\sqrt{-1}\ ##, you have

##\ B - B^* = 2j\text{ Im}(B) \ ##
 
  • #5
jbunniii
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Sorry, I just couldn't bring myself to type ##j##. :tongue:
 
  • #6
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Sorry, I just couldn't bring myself to type ##j##. :tongue:
I know what you mean. Also, when I saw j2 in the first post, I first thought he meant j2. I like 2j better than j2, and 2i better than 2j.
 
  • #7
Ray Vickson
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Homework Statement



Find the partial fractions expansion in the following form,

[tex]G(s) = \frac{1}{(s+1)(s^{2}+4)} = \frac{A}{s+1} + \frac{B}{s+j2} + \frac{B^{*}}{s-j2}[/tex]

Homework Equations





The Attempt at a Solution



I expanded things out and found the following,

[tex]1 = A(s^{2} + 4) + B(s^{2} + (1-j2)s -j2) + B^{*}(s^{2} + (1+j2)s + j2)[/tex]

From this I get the following equations,

[tex]A + B + B^{*} = 0[/tex]

[tex]B(1-j2) + B^{*}(1+j2) = 0[/tex]

[tex]4A - Bj2 + B^{*}j2 = 1[/tex]

This doesn't seem like a pleasant set of equations to solve.

I did another partial fractions expansion like so,


[tex]G(s) = \frac{1}{(s+1)(s^{2}+4)} = \frac{D}{s+1} + \frac{Es + F}{s^{2}+4}[/tex]

and found,

[tex]D = \frac{1}{5}, E = - \frac{1}{5}, F = \frac{1}{5}[/tex]

but this doesn't give me the form I need.

Any ideas? Any convenient ways to solve this?

It is probably easier to first expand as
[tex] \frac{1}{(s+1)(s^2+4)} = \frac{A}{s+1} + \frac{Bs+C}{s^2+4}[/tex]
then expand
[tex] \frac{1}{s^2+4} = \frac{E}{s+2i} + \frac{F}{s-2i}[/tex]
BTW: in TeX (or LaTeX) you do not need to write 's^{2}'; just plain 's^2' will do. You only need the '{.}' if the super-script (or sub-script) is more than one term. (Even a multi-letter single command can do without the '{.}'; for example, look at ##s^\alpha##, which was entered as s^\alpha.)
 
  • #8
Ray Vickson
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Homework Statement



Find the partial fractions expansion in the following form,

[tex]G(s) = \frac{1}{(s+1)(s^{2}+4)} = \frac{A}{s+1} + \frac{B}{s+j2} + \frac{B^{*}}{s-j2}[/tex]

Homework Equations





The Attempt at a Solution



I expanded things out and found the following,

[tex]1 = A(s^{2} + 4) + B(s^{2} + (1-j2)s -j2) + B^{*}(s^{2} + (1+j2)s + j2)[/tex]

From this I get the following equations,

[tex]A + B + B^{*} = 0[/tex]

[tex]B(1-j2) + B^{*}(1+j2) = 0[/tex]

[tex]4A - Bj2 + B^{*}j2 = 1[/tex]

This doesn't seem like a pleasant set of equations to solve.

I did another partial fractions expansion like so,


[tex]G(s) = \frac{1}{(s+1)(s^{2}+4)} = \frac{D}{s+1} + \frac{Es + F}{s^{2}+4}[/tex]

and found,

[tex]D = \frac{1}{5}, E = - \frac{1}{5}, F = \frac{1}{5}[/tex]

but this doesn't give me the form I need.

Any ideas? Any convenient ways to solve this?

It is probably easier to first expand as
[tex] \frac{1}{(s+1)(s^2+4)} = \frac{A}{s+1} + \frac{Bs+C}{s^2+4}[/tex]
then expand
[tex] \frac{1}{s^2+4} = \frac{E}{s+2i} + \frac{F}{s-2i}[/tex]
BTW: in TeX (or LaTeX) you do not need to write 's^{2}'; just plain 's^2' will do. You only need the '{.}' if the super-script (or sub-script) is more than one term. (Even a multi-letter single command can do without the '{.}'; for example, look at ##s^\alpha##, which was entered as s^\alpha.
 

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