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UnkleRhaukus
Group Title
Use transform definitions, and the evaluation
of a suitable double integral, to calculate the
Laplace transform of the following integral
\[\mathcal L\left\{\int\limits_0^tx(u)\cdot\text du\right\}\]
 2 years ago
 2 years ago
UnkleRhaukus Group Title
Use transform definitions, and the evaluation of a suitable double integral, to calculate the Laplace transform of the following integral \[\mathcal L\left\{\int\limits_0^tx(u)\cdot\text du\right\}\]
 2 years ago
 2 years ago

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UnkleRhaukus Group TitleBest ResponseYou've already chosen the best response.0
\[\begin{align*}\mathcal L\left\{\int\limits_0^tx(u)\cdot\text du\right\} &=\int\limits_0^\infty\left(\int\limits_0^tx(u)\cdot\text du\right)e^{pt}\cdot\text dt\\ &=\\ &=\\ &=\end{align*}\]
 2 years ago

wio Group TitleBest ResponseYou've already chosen the best response.0
I'm guessing that this involve integration by parts...
 2 years ago

UnkleRhaukus Group TitleBest ResponseYou've already chosen the best response.0
do i have to reverse the order of integration ?
 2 years ago

wio Group TitleBest ResponseYou've already chosen the best response.0
Not quite sure though.
 2 years ago

UnkleRhaukus Group TitleBest ResponseYou've already chosen the best response.0
@Zarkon
 2 years ago

UnkleRhaukus Group TitleBest ResponseYou've already chosen the best response.0
\[\begin{align*}\mathcal L\left\{\int\limits_0^tx(u)\cdot\text du\right\} &=\int\limits_0^\infty\left(\int\limits_0^tx(u)\cdot\text du\right)e^{pt}\cdot\text dt\\ &=\int\limits_0^\infty\int\limits_0^ux(u)e^{pt}\cdot\text dt\cdot\text du\\ ?&=\int\limits_0^\infty x(u)\int\limits_0^ue^{pt}\cdot\text dt\cdot\text du\\ \end{align*}\]
 2 years ago

UnkleRhaukus Group TitleBest ResponseYou've already chosen the best response.0
@mahmit2012
 2 years ago

wio Group TitleBest ResponseYou've already chosen the best response.0
Somehow u become a t
 2 years ago

UnkleRhaukus Group TitleBest ResponseYou've already chosen the best response.0
dw:1353224604904:dw
 2 years ago

UnkleRhaukus Group TitleBest ResponseYou've already chosen the best response.0
have to reverse the order of integration correctly ?
 2 years ago

wio Group TitleBest ResponseYou've already chosen the best response.0
is u still constant with respect to t then?
 2 years ago

UnkleRhaukus Group TitleBest ResponseYou've already chosen the best response.0
i think so
 2 years ago

wio Group TitleBest ResponseYou've already chosen the best response.0
Well, if we say that \(X(t)\) is the antiderivative of \(x(t)\)....then \[ X(t) = \int_0^tx(u)\cdot du \]and \(X'(t)=x(t)\) So the antiderivative is going to be dependent on \(t\) and yet the derivative is constant...?
 2 years ago

wio Group TitleBest ResponseYou've already chosen the best response.0
Is there anyway to verify the answer you're getting at?
 2 years ago

UnkleRhaukus Group TitleBest ResponseYou've already chosen the best response.0
wolfram tells me the answer is \[\frac{\mathcal L\{x(t)\}}{p}=\frac1p\int\limits_0^\infty x(t)e^{pt}\text dt\]
 2 years ago

wio Group TitleBest ResponseYou've already chosen the best response.0
Okay nevermind what I said before, it looks like what you are doing is correct.
 2 years ago

wio Group TitleBest ResponseYou've already chosen the best response.0
What are you getting for that inner integral?
 2 years ago

UnkleRhaukus Group TitleBest ResponseYou've already chosen the best response.0
\[\begin{align*}&=\int\limits_0^\infty x(u)\int\limits_0^ue^{pt}\cdot\text dt\cdot\text du\\ &=\int\limits_0^\infty x(u) \frac{e^{pt}}{p}\Big_0^u\cdot\text du\\ &=\int\limits_0^\infty x(u) \frac{1e^{pu}}p\cdot\text du\\\end{align*}\]
 2 years ago

UnkleRhaukus Group TitleBest ResponseYou've already chosen the best response.0
i think i have made a mistake but im not sure where
 2 years ago

mahmit2012 Group TitleBest ResponseYou've already chosen the best response.1
dw:1353228810868:dw
 2 years ago

mahmit2012 Group TitleBest ResponseYou've already chosen the best response.1
dw:1353228991552:dw
 2 years ago

UnkleRhaukus Group TitleBest ResponseYou've already chosen the best response.0
H(tu) is the Heaviside unit step function?
 2 years ago

mahmit2012 Group TitleBest ResponseYou've already chosen the best response.1
dw:1353229734181:dw
 2 years ago

UnkleRhaukus Group TitleBest ResponseYou've already chosen the best response.0
\[\begin{align*}\mathcal L\left\{\int\limits_0^tx(u)\cdot\text du\right\} &=\int\limits_0^\infty\left(\int\limits_0^tx(u)\cdot\text du\right)e^{pt}\cdot\text dt\\ &=\int\limits_0^\infty\int\limits_0^\infty x(u)H(tu)\cdot\text du\cdot e^{pt}\cdot\text dt\\ &=\int\limits_0^\infty\int\limits_0^\infty x(u)H(tu)e^{pt}\cdot\text dt\cdot\text du\\ &=\int\limits_0^\infty x(u)\int\limits_0^\infty H(tu)e^{pt}\cdot\text dt\cdot\text du\\ &=\int\limits_0^\infty x(u)\mathcal L \Big\{H(tu)\Big\}\cdot\text du\\ &=\int\limits_0^\infty x(u)\frac{e^{up}}{p}\cdot\text du\\ &=\frac1p\int\limits_0^\infty x(u)e^{up}\cdot\text du\\ &=\frac{X(p)}p\color{red}\checkmark \\ \end{align*}\] Thankyou mahmit2012!
 2 years ago
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