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How to integrate a fraction of sums of exponentials?

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DRJP
#1
Dec21-11, 07:20 AM
P: 1
Is it possible to have an solution to this sort of integral? And if not, why not?

[tex] \int_0^\infty \frac{e^{-ax}}{e^{-bx}+e^{-cx}}dx [/tex]

Is a Taylor expansion the only way forward?

Many thanks
David
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dextercioby
#2
Dec21-11, 07:29 AM
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Quote Quote by DRJP View Post
Is it possible to have an solution to this sort of integral? And if not, why not?

[itex]\int_0^\infty \frac{e^{-ax}}{e^{-bx}+e^{-cx}}dx[/itex]

Is a Taylor expansion the only way forward?

Many thanks
David
Use [tex ] instead of inline tex if you're not writing a formula on the same line with words.

[tex]\int_0^\infty \frac{e^{-ax}}{e^{-bx}+e^{-cx}}dx[/tex]

looks better and is easier to read.

As for your question, before jumping to series expansions and substitutions, specify if the arbitrary constants are positive or negative. This makes a huge difference on the final result.
Then try to get rid of as many exponentials as possible. You can make the substitution (a,b,c >0) [itex] \displaystyle{e^{-ax}} = t [/itex] and see what you get.


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