00:01
We have two tanks with salt water flowing in between them.
00:06
Each one started out with 100 gallons.
00:09
Tank a started out with 95 pounds of salt, and tank b started out with 55.
00:14
The total volume is 200 gallons, although it's pumped in between the tanks, and you can see that it goes faster from b to a than it goes from a to b.
00:26
I'm going to use x to represent the number of pounds of salt.
00:34
So first, we want to find the differential equation satisfied by the pounds of salt.
00:46
So there's the one for xa.
00:50
So basically the change in the amount of salt in tank a is the amount coming in from b minus the amount leaving a.
01:03
And then in her tank b, it's kind of the reverse.
01:12
The amount of salt in b, or the change in the amount of salt in b is the amount coming in from a minus the amount going out to b.
01:25
If you notice, the right -hand sides of those two equations are equal and opposite.
01:31
So if i add them together, i get zero.
01:35
But first of all, you know that xa of zero is 95.
01:39
Pounds and xb of zero is 55 pounds that's given to us in part b you want a relationship between xa and xb that doesn't rely on the differential equations necessarily and that is that the sum of them has to be 155 that there's no salt entering or leaving and that's the point of question c the system is closed so no salt enters or leave so the total amount of salt has to be constant...