# Another ODE

• Dec 17th 2009, 04:48 AM
WannaBe
Another ODE
I've attached the relevant pictures. The question is:
Let X,Y be two containers.
At t=0, container X has 100 lt. of water with 2 kg of salt in it and Y has 100 lt. of water with 6 kg of salt.
On each t>0, the system transports water as the you can see in the picture.

In each minute t, let x(t), y(t) be the quantities of salt in X,Y in kg's.
t is measured in minutes!

You should notice that on each time, there are excatly 100 lt. in each container!

Write an ODE that gives the quantity of salt on each container as a function of time, solve it and calculate how much kg's of salt will be in the container after 10 minutes from the start of the process.
My attemt:

I wrote the equations this way:
x'(t)= -8x(t)/100 +2y(t)/100
y'(t) = 8x(t)/100 -8y(t)/100

We get the ODE: w' =Aw ... The eignvalues of A are: -4/100 and -12/100 ...
After I solve these two equations I get two soloutions- one for x(t) and one for y(t)...The only problem is that these soloutions don't match the data of the question...
HELP is needed!

TNX
• Dec 17th 2009, 09:53 AM
Calculus26
Those are the correct eigenvalues

Eigen vectors are for -1/25 (-1/2,1)

-3/25 (1/2,1)

applying the ICs

x= 5/2*e^(-1/25t) -1/2 e^(-25/t)

y= e^(-1/25t) + e^(-3/25t)

See attachment
• Dec 17th 2009, 11:49 AM
WannaBe
Wow, so I was right... TNX a lot for your verification!