Laplace time differentiation property

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Discussion Overview

The discussion revolves around proving the time differentiation property of the Laplace transform, specifically the relationship between the derivative of a function in the time domain and its representation in the Laplace domain. Participants explore the steps involved in applying the Laplace transform to the derivative and the implications of different integration limits.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant suggests starting with the Laplace transform of the derivative, L[dx/dt], and using the definition of the Laplace transform.
  • Another participant expresses uncertainty about how to handle a term in the integral that should vanish, proposing the Final Value Theorem as a possible solution.
  • A later reply emphasizes that the variable of integration is t and suggests taking terms independent of t outside the integral.
  • There is a discussion about the integration limits, with one participant noting that the standard limits are from 0 to +∞, while questioning the use of -∞ to +∞ in their class.
  • Participants agree that the final integral results in sX(s), but there is uncertainty about the behavior of the preceding term.

Areas of Agreement / Disagreement

Participants generally agree on the final result of the integral being sX(s), but there is disagreement and uncertainty regarding the treatment of the preceding term and the correct integration limits.

Contextual Notes

There are unresolved questions about the assumptions regarding integration limits and the application of the Final Value Theorem. The discussion reflects different interpretations of the Laplace transform definitions.

khdani
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Hello,
I'm trying to prove the time differentiation property of Laplace transform.
dx(t)/dt = sX(s)


http://img10.imageshack.us/img10/290/tlaplace.jpg http://g.imageshack.us/img10/tlaplace.jpg/1/

how do i continue from here ?
 
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I'm not sure what the first step here is supposed to represent. Wouldn't the first thing to do be to write

L[dx/dt] = ...


And then use the definition of the Laplace transform?
 
yes, you right
the first term should be like you said
but what i have to do with the first term of the last part?
it should vanish but i don't know how? maybe by Final Value Theorem ?
 
In the final integral, t is the variable of integration.

Take any term that is independent of t outside of the integral, and see what you get.

p.s. your integration limits are wrong.
 
the final integral is sX(s)
but the one before should vanish somehow..., i think it's because of Final Value Theorem
why do you think the integration limits are wrong?
 
khdani said:
the final integral is sX(s)
Yes, good.

... but the one before should vanish somehow..., i think it's because of Final Value Theorem
why do you think the integration limits are wrong?

It looks like there are two definitions of the Laplace transform:
http://mathworld.wolfram.com/LaplaceTransform.html

The integrations limits are usually taken from 0 to +∞, but maybe your class is using the -∞ to +∞ definition instead. I didn't know about that definition before.

If the limits are 0 to +∞, you can just evaluate the x(t) e-st term at the limits. I'm not sure what to do if you're using the -∞ definition though.
 

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