D.E. Linear equation with integrating factor

1. Feb 7, 2015

Marco Lugo

1. The problem statement, all variables and given/known data

https://webwork.utpa.edu/webwork2_files/tmp/equations/2d/02a7e6a06f5b2424758fa01cc965f71.png [Broken] with https://webwork.utpa.edu/webwork2_files/tmp/equations/80/81c176aa8964438a63eb096513245f1.png [Broken]

2. Relevant equations

Standard form: y' + p(x)y = f(x)

Integrating factor: r(x)= e∫p(x)dx

Solution: y(x) = e-∫p(x)dx(∫e∫p(x)dx ⋅ f(x) dx + C)

3. The attempt at a solution

From, y' - y = 7et + 56e8t

I got, p(x) = -1 and f(x) = 7et + 56e8t

So the integrating factor is r(t)= e∫-1 dt = e-t

Plug it into the solution formula,
y(t)= et [∫e-t (7et + 56e8t) dt + C]

= et (7t + 8e7t+ C)

= 7tet + 8e8t+ Cet

Using the initial condition y(0)= 2

= 8 + C = 2
C = -6

My final solution,
y(t) = 7tet + 8e8t- 6et

check: y(0) = 7(0)e(0) + 8e8(0)- 6e(0) = 2

But when I type it into WeBWork, it says my answer is incorrect. I don't think its a syntax problem because it has a preview button that allows you to see your actual problem.

Could it be that, y' - y = 7et + 56e8t is not in standard form?

Last edited by a moderator: May 7, 2017
2. Feb 7, 2015

Staff: Mentor

I don't see anything wrong with your work or your solution. You can (and should) verify for yourself that your solution is correct by first checking that the initial condition is satisfied, and then by substituting your solution into the differential equation. Your solution should make the DE a true statement.

As far as why the software doesn't accept your solution, make sure that you are working the same problem it thinks you are working. I would also check with the instructor.

Last edited by a moderator: May 7, 2017
3. Feb 7, 2015

Marco Lugo

I checked the initial condition but you're right I should have substituted my solution into the problem.

my solution,
y(t) = 7tet + 8e8t - 6et

check:
dy/dt= 64e8t + 7tet + et

substituting both into the initial probelm,

dy/dt - y = 56e8t + 7et

64e8t + 7tet + et - 7tet - 8e8t + 6et

I get, 56e8t + 7et

I don't see anything wrong. I would submit the answer again but I only have 2 more tries, so I want to get some more advice before I do.

4. Feb 7, 2015

Ray Vickson

Is there some special format you must adhere to when submitting solutions? For example, do you need to say exp(t) instead of et? Does it matter whether the terms come in a particular order, different from the one you wrote? For example, would $8 e^{8t} + 7t e^{t} - 6e^{t}$ or $8 e^{8t} - 6 e^{t} + 7 t e^{t}$ both be acceptable? Can you write $t e^{t}$ (or $t \exp(t)$), or must it be in the form $e^{t} \, t$ (or $\exp(t)\, t$), etc? I would certainly hope that the software would not be so picky, but who knows?

5. Feb 7, 2015

Marco Lugo

It worked! I had to use the exp(t) notation; which is weird because the software usually isn't picky and it would say if it wanted something expressed in a certain way.
Anyway thanks Mark44 and Ray Vickson for all you're help :D