Laplace Transform L[x(t)] given, find L[tx(t)]

In summary: Is the answer.In summary, the problem asks for the Laplace transform of a function given the Laplace transform of another function. It also asks for the derivative of the first function with respect to the second function. Simplifying the rest of the equation yields the answer.
  • #1
Color_of_Cyan
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Homework Statement



If L[x(t)] = (s + 4)/(s2 + 1), find L[tx(t)]

Homework Equations


Laplace transform:

F(s) = 0∫ f(t)e-stdtLaplace table

The Attempt at a Solution


Clearly it's not just asking for a Laplace transform. Not sure what it's specifically asking to be honest.

t multiplied by whatever is inside the equation definitely isn't the answer.
 
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  • #2
Color_of_Cyan said:

Homework Statement



If L[x(t)] = (s + 4)/(s2 + 1), find L[tx(t)]

Homework Equations


Laplace transform:

F(s) = 0∫ f(t)e-stdtLaplace table

The Attempt at a Solution


Clearly it's not just asking for a Laplace transform.
Clearly it is. There are a couple of approaches you could take, but I'd like to see what you have tried before I share them with you.
Color_of_Cyan said:
Not sure what it's specifically asking to be honest.

t multiplied by whatever is inside the equation definitely isn't the answer.
I don't know what this means...
 
  • #3
Color_of_Cyan said:
Not sure what it's specifically asking to be honest.

Is it asking for the Laplace transform of the function ##t x(t)## (i.e., the function obtained by multiplying ##x(t)## by ##t##), given the Laplace transform of ##x(t)##? That's what it looks like to me, but I'd like to make sure you have transcribed the problem statement correctly.

Also, please show us explicitly your attempt at a solution(i.e., with equations showing the steps of the calculation you attempted). Just describing it in words isn't enough.
 
  • #4
Okay, there seems to be one property that sticks out for this:

L[tf(t)] = -dF(s)/ds

L[x(t)] = (s+4)/(s2 + 1)

L[f(t)] = (s+4)/(s2 + 1). Then find L[tf(t)]

$$\frac{d} {ds} [\frac {s+4} {s^2 + 1}] $$

Just this derivative?

= (d/ds)[(s+4)/(s2 + 1)-1]

=
$$ \frac {(1)(s^2 + 1) - (2s)(s+4)} {{(s^2 + 1)}^2} $$

Would I need only simplify the rest of this to get L[tx(t)] ?
 
  • #5
Color_of_Cyan said:
Okay, there seems to be one property that sticks out for this:

L[tf(t)] = -dF(s)/ds

L[x(t)] = (s+4)/(s2 + 1)

L[f(t)] = (s+4)/(s2 + 1). Then find L[tf(t)]

$$\frac{d} {ds} [\frac {s+4} {s^2 + 1}] $$

Just this derivative?

= (d/ds)[(s+4)/(s2 + 1)-1]

=
$$ \frac {(1)(s^2 + 1) - (2s)(s+4)} {{(s^2 + 1)}^2} $$

Would I need only simplify the rest of this to get L[tx(t)] ?
Yes
 
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Likes Color_of_Cyan
  • #6
Okay, thanks! :)

Edit: Oh wait I forgot that there was a negative sign by the derivative, so

(s2 + 8s - 1)/(s2 + 1)2
 
Last edited:

FAQ: Laplace Transform L[x(t)] given, find L[tx(t)]

1. What is the Laplace transform?

The Laplace transform is a mathematical tool used to solve differential equations. It transforms a function of time into a function of complex frequency, making it easier to solve equations involving derivatives.

2. How is the Laplace transform calculated?

The Laplace transform is calculated using an integral formula, which involves multiplying the function by an exponential term and integrating from 0 to infinity.

3. What does L[x(t)] given mean?

L[x(t)] given refers to the original function being transformed. In other words, x(t) is the function that is being transformed using the Laplace transform.

4. What is the significance of the Laplace transform of tx(t)?

The Laplace transform of tx(t) represents the first derivative of the original function x(t). This can be seen as a way to "move" the derivative from the time domain to the frequency domain, making it easier to solve differential equations.

5. How is the Laplace transform used in real-world applications?

The Laplace transform has various applications in engineering, physics, and other fields. It is commonly used to solve differential equations in control systems, circuit analysis, and signal processing. It is also used in the study of heat transfer and vibrations.

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