anonymous
  • anonymous
write a differential equation in dy/dt=ay+b form with all solutions approaching y=3 as t-> infinity
Mathematics
  • Stacey Warren - Expert brainly.com
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SOLVED
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jamiebookeater
  • jamiebookeater
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anonymous
  • anonymous
no im dumb
anonymous
  • anonymous
Actually, we can reason it out another way.
anonymous
  • anonymous
Do you know what dy/dt *means*?

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zzr0ck3r
  • zzr0ck3r
y = (3n+1)/(n+1) dy/dt = 2/(x+1)^2
anonymous
  • anonymous
ya i do
anonymous
  • anonymous
Okay then, so if limt->infy should give y(t) = 3, then, at infinity, what should the slope of y(t) be?
anonymous
  • anonymous
3?
anonymous
  • anonymous
If, at infinity, our function needs to keep getting closer to y=3, then what value should the SLOPE of the function be approaching?
anonymous
  • anonymous
infinity
anonymous
  • anonymous
wait no 0
zzr0ck3r
  • zzr0ck3r
what is the limit of y = (3n+1)/(n+1) at infinity?
anonymous
  • anonymous
@v4xN0s Yes, 0. What's another name for slope?
anonymous
  • anonymous
derivative
anonymous
  • anonymous
Yes, or, more relavant to this problem, dy/dt. Now, as you said, we want dy/dt = 0 when y = 3, right?
anonymous
  • anonymous
yep
anonymous
  • anonymous
Okay then. In ay+b, if I make y = 3, then what can a and b be?
anonymous
  • anonymous
so it would be y'=3-y?
anonymous
  • anonymous
That's one solution, b = 3 and a = -1.
zzr0ck3r
  • zzr0ck3r
no we want y = 3 when t = infinity?
anonymous
  • anonymous
@zzr0ck3r yes thats what the question says
anonymous
  • anonymous
what if all the solutions diverged from y=2
anonymous
  • anonymous
"diverged from y = 2" --> what?
anonymous
  • anonymous
as t approaches infinity
anonymous
  • anonymous
@v4xN0s we're not done with the previous one yet
anonymous
  • anonymous
oh alright i thought that was the write answer QQ
anonymous
  • anonymous
QQ? Anyway, that's just ONE anwer. In fact, any a and b that makes 3a+b = 0 works.
anonymous
  • anonymous
But it depends on the initial conditions, too.
anonymous
  • anonymous
oh i see well i just needed one diff eq for the problem
anonymous
  • anonymous
Okay, your example of dy/dt=3-y was a bit of a lucky guess.
anonymous
  • anonymous
See, if your initial condition is\[y_0>3\]then your slope is negative, and y decreases until it 'reaches' 3, at which point it 'stays' constant.
anonymous
  • anonymous
i see and if its less than 3 then its positive and increases as it gets closer to 3
anonymous
  • anonymous
Yes. If\[y_0=3\], then your solution is a line, starting at y=3 and staying that way (dy/dt = 0).
anonymous
  • anonymous
wait no slope still decreases
anonymous
  • anonymous
Um no.....
anonymous
  • anonymous
|dw:1345523728600:dw|
anonymous
  • anonymous
Yeah, those are the three possible solution curves. Why do you think the slope decreases for the initial cond less than 3 one?
anonymous
  • anonymous
the bottom curve's slope is appreaching 0 isnt it, so the slope would be decreasing
anonymous
  • anonymous
Yes, its *decreasing* but *positive*. That's the whole deal - regardless where it is, it gets closer to 0 as it goes to infinity.
anonymous
  • anonymous
ok got that one, but what if solutions diverge from y=2
anonymous
  • anonymous
So, back to where I was. You were lucky when you picked dy/dt as 3-y. What would have happened if you've done y-3 (note, this still has slope of 0 when y =3)?
anonymous
  • anonymous
it becomes negative?
anonymous
  • anonymous
Well think about it. If our \[y_0>3\]What will happen?
anonymous
  • anonymous
We're above the line we want to tend to in end behavior, and what's our slope?
anonymous
  • anonymous
its decreasing and negative
anonymous
  • anonymous
No. if y>3, and dy/dt is y-3, our slope is positive. As y increases, y-3 increases. Thus dy/dt is increasing and positive. What do you think this will do to the function? Then try y_0=3 and y_0<3.
anonymous
  • anonymous
sorry but i have no idea
anonymous
  • anonymous
Well, if y > 3, do you see why y-3 is > 0? Then dy/dt is greater than 0. Thus our slope is positive. Our particle or whatever increases in y in that differential amount of time, resulting in an even larger slope (since y-3 also gets bigger). Do you not see this makes you diverge?

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