Can't Figure the Algebra

  • #1
3,003
6

Homework Statement



I at a point in a derivation where I have the expression:

[tex]A = \frac{e^{mx} - e^{2ml - mx}}{1-e^{2ml}}[/tex]

I have double checked my work leading up to this point, so i am confident my expression for 'A' is correct. I am supposed to reduce it to

[tex]A = e^{mx}[/tex]

but I am not seeing the trick here. I have tried numerous approaches from factoring the denominator and various arrangements of the numerator. I have a feeling it is one of those random tricks i need. Any thoughts?
 
Last edited:

Answers and Replies

  • #2
35,287
7,140

Homework Statement



I at a point in a derivation where I have the expression:

[tex]A = \frac{e^{mx} - e^{2ml - mx}}{1-e^{2ml}}[/tex]
I think you might have a sign error in the exponent on the first term in the numerator.

Assuming this is the case for the moment, you have
[tex]A = \frac{e^{-mx} - e^{2ml - mx}}{1-e^{2ml}}[/tex]
[tex]= \frac{e^{-mx} - e^{2ml}\cdot e^{ -mx}}{1-e^{2ml}}[/tex]
[tex]= \frac{e^{-mx}(1 - e^{2ml})}{1-e^{2ml}} = e^{-mx}[/tex]
I have double checked my work leading up to this point, so i am confident my expression for 'A' is correct. I am supposed to reduce it to

[tex]A = e^{-mx}[/tex]

but I am not seeing the trick here. I have tried numerous approaches from factoring the denominator and various arrangements of the numerator. I have a feeling it is one of those random tricks i need. Any thoughts?
 
  • #3
3,003
6

Homework Statement



I at a point in a derivation where I have the expression:

[tex]A = \frac{e^{mx} - e^{2ml - mx}}{1-e^{2ml}}[/tex]

I have double checked my work leading up to this point, so i am confident my expression for 'A' is correct. I am supposed to reduce it to

[tex]A = e^{-mx}[/tex]

but I am not seeing the trick here. I have tried numerous approaches from factoring the denominator and various arrangements of the numerator. I have a feeling it is one of those random tricks i need. Any thoughts?



I'm sorry it's supposed to come out to be a positive exponent. That is,

[tex]A = e^{mx}[/tex]

I have edited OP.
 
  • #4
35,287
7,140
In that case, I think your error is in the second term in the numerator.
[tex]A = \frac{e^{mx} - e^{2ml + mx}}{1-e^{2ml}}[/tex]

[tex]= \frac{e^{mx} - e^{2ml}\cdot e^{ mx}}{1-e^{2ml}}[/tex]

[tex]= \frac{e^{mx}(1 - e^{2ml})}{1-e^{2ml}} = e^{mx}[/tex]
 

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