# differential equations problem

• Oct 3rd 2009, 11:52 AM
morganfor
differential equations problem
I haven't dealt with differential equations in a while so I completely forget how to do this problem.

Given dx/dt=y and dy/dt=x

Verify that f(t)=Ce^t+De^-t and g(t)=Ce^t - De^-t give a solution to the system of differential equations, where C and D are arbitrary constants.

Thanks!
• Oct 3rd 2009, 01:13 PM
DeMath
Quote:

Originally Posted by morganfor
I haven't dealt with differential equations in a while so I completely forget how to do this problem.

Given dx/dt=y and dy/dt=x

Verify that f(t)=Ce^t+De^-t and g(t)=Ce^t - De^-t give a solution to the system of differential equations, where C and D are arbitrary constants.

Thanks!

Have уou to solve a system of diff. equations $\left\{ \begin{gathered}\frac{{dx}}{{dt}} = y, \hfill \\\frac{{dy}}{{dt}} = x. \hfill \\ \end{gathered} \right.$ ?

Differentiate, for example, the first equation $\frac{{{d^2}x}}
{{d{t^2}}} - \frac{{dy}}{{dt}} = 0$
and because $\frac{dy}{dt} = x$
you get a simple diff. equation $x''\left( t \right) - x\left( t \right) = 0$

I think you know what you need to do next.
• Oct 3rd 2009, 01:53 PM
morganfor
Actually, I'm having some trouble differentiating x''-x=0. I can't quite seem to get the e's in my equation.
• Oct 4th 2009, 12:06 AM
mr fantastic
Quote:

Originally Posted by morganfor
Actually, I'm having some trouble differentiating x''-x=0. I can't quite seem to get the e's in my equation.

$\frac{d^2 x}{dt^2} - x = 0$ is a linear second order differential equation with constant coefficients. Have you studied that type of DE before?
• Oct 4th 2009, 07:22 AM
morganfor
I'm sure I have but it's been atleast 3 years I just need a little refresher
• Oct 5th 2009, 02:16 AM
The Second Solution
Quote:

Originally Posted by morganfor
I'm sure I have but it's been atleast 3 years I just need a little refresher

Have you tried using Google to get that refresher? Keywords:

linear second order differential equation constant coefficients
• Oct 5th 2009, 06:25 AM
morganfor
yes i did actually - i forgot to add that to the thread earlier.

thanks everyone!
• Oct 5th 2009, 06:39 AM
garymarkhov
Quote:

Originally Posted by morganfor
yes i did actually - i forgot to add that to the thread earlier.

thanks everyone!

Morganfor, the differential equations videos at Khan Academy are excellent if you're looking for a refresher. Good luck :)
• Oct 5th 2009, 08:16 AM
HallsofIvy
This problem does not ask you to solve the equations, just show that the given functions satisfy the equations. That's much simpler.
If $x(t)=Ce^t+De^{-t}$ and $y(t)=Ce^t - De^{-t}$, what are dx/dt and dy/dt?
• Oct 5th 2009, 04:46 PM
morganfor
i apologize that i'm still stuck on this problem!

as far as verifying that the given solutions are correct - I'm sure I'm missing a simple step but I can't seem to differentiate f(t)=Ce^t+De^-t and g(t)=Ce^t-De^t to give me y and x. I'm confused as to which substitutions I need to make...