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#### karush

##### Well-known member

- Jan 31, 2012

- 2,886

- Thread starter karush
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- Jan 31, 2012

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- #3

- Jan 31, 2012

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$\quad\displaystyle \frac{dy}{dt}=2y-5, \quad y(0)=y_0$

rewrite

$\quad y'-2y=-5$

obtain u(x)

$\quad u(x)=\exp\int-2\, dx = e^{-2t}$

then

$\quad (e^{-2t}y)'=-5e^{-2t}$

integrate both sides

$\quad\displaystyle e^{-2t}y=-5\int e^{-2t} dt=-5e^{-2t}+c_1$

finally

$\quad\displaystyle y

=-5\frac{e^{-2t}}{e^{-2t}}+\frac{c_1}{e^{-2t}}=c_1e^{2t}-5$

so at $\quad y(0)=y_0$

ok I don't see how they got this answer

rewrite

$\quad y'-2y=-5$

obtain u(x)

$\quad u(x)=\exp\int-2\, dx = e^{-2t}$

then

$\quad (e^{-2t}y)'=-5e^{-2t}$

integrate both sides

$\quad\displaystyle e^{-2t}y=-5\int e^{-2t} dt=-5e^{-2t}+c_1$

finally

$\quad\displaystyle y

=-5\frac{e^{-2t}}{e^{-2t}}+\frac{c_1}{e^{-2t}}=c_1e^{2t}-5$

so at $\quad y(0)=y_0$

ok I don't see how they got this answer

Last edited:

- Aug 30, 2012

- 1,180

First, what is your c1 in terms of \(\displaystyle y_0\)? You never finished that part.$\quad\displaystyle \frac{dy}{dt}=2y-5, \quad y(0)=y_0$

rewrite

$\quad y'-2y=-5$

obtain u(x)

$\quad u(x)=\exp\int-2\, dx = e^{-2t}$

then

$\quad (e^{-2t}y)'=-5e^{-2t}$

integrate both sides

$\quad\displaystyle e^{-2t}y=-5\int e^{-2t} dt=-5e^{-2t}+c_1$

finally

$\quad\displaystyle y

=-5\frac{e^{-2t}}{e^{-2t}}+\frac{c_1}{e^{-2t}}=c_1e^{2t}-5$

so at $\quad y(0)=y_0$

ok I don't see how they got this answer

Also take a look at this:

\(\displaystyle y = 5 +(y_0 - 5)e^{-t}\)

\(\displaystyle y' = -(y_0 - 5)e^{-t}\)

Thus

\(\displaystyle y' - 2y = -(y_0 - 5)e^{-t} - 2(5 + (y_0 - 5)e^{-t}) = -10 - 3(y_0 - 5)e^{-t} \neq -5\)

So the given solution does not match the differential equation. You have the skill to do this check. (And you should always check your own solutions anyway.)

-Dan

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- #5

\(\displaystyle \frac{d}{dt}\left(e^{-2t}y\right)=-5e^{-2t}\)

Integrate using the boundaries:

\(\displaystyle \int_{y_0}^{e^{-2t}y}\,du=-5\int_{0}^{t}e^{-2v}\,dv\)

\(\displaystyle e^{-2t}y-y_0=\frac{5}{2}\left(e^{-2t}-1\right)\)

\(\displaystyle y-y_0e^{2t}=\frac{5}{2}\left(1-e^{2t}\right)\)

\(\displaystyle y=\left(y_0-\frac{5}{2}\right)e^{2t}+\frac{5}{2}\)

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- #6

- Jan 31, 2012

- 2,886

rewrite

$\quad y'-2y=-5$

obtain u(x)

$\quad u(x)=\exp\int-2\, dx = e^{-2t}$

then

$\quad (e^{-2t}y)'=-5e^{-2t}$

integrate both sides

$\quad\displaystyle e^{-2t}y=-5\int e^{-2t} dt=-5e^{-2t}+c_1$

finally

$\quad\displaystyle y

=-5\frac{e^{-2t}}{e^{-2t}}+\frac{c_1}{e^{-2t}}=c_1e^{2t}-5$

so at $\quad y(0)=y_0$

$\quad\displaystyle y(0)=-5+c_1=y_0 $

then

$\quad\displaystyle c_1=y_0+5$

- Thread starter
- #7

- Jan 31, 2012

- 2,886

\(\displaystyle \frac{d}{dt}\left(e^{-2t}y\right)=-5e^{-2t}\)

Integrate using the boundaries:

\(\displaystyle \int_{y_0}^{e^{-2t}y}\,du=-5\int_{0}^{t}e^{-2v}\,dv\)

\(\displaystyle e^{-2t}y-y_0=\frac{5}{2}\left(e^{-2t}-1\right)\)

\(\displaystyle y-y_0e^{2t}=\frac{5}{2}\left(1-e^{2t}\right)\)

\(\displaystyle y=\left(y_0-\frac{5}{2}\right)e^{2t}+\frac{5}{2}\)

well that is pretty cool .... better than the book process

but I still don't think the boundary thing has registered with me

- Aug 30, 2012

- 1,180

When solving differential equations you see a lot of arbitrary constants. (ie. there are many solutions to the same differential equation.) The boundary conditions simply give you information about those constants. In this case you had c1 as a constant after solving the differential equation. The boundary condition \(\displaystyle y(0) = y_0\) tells you what c1 is in terms of \(\displaystyle y_0\). Perhaps it would be more meaningful to you if we specified y(0) = 3 or something like that.well that is pretty cool .... better than the book process

but I still don't think the boundary thing has registered with me

-Dan