# Simple DE....but lack of practice.

• August 28th 2010, 05:43 AM
JavaJunkie
Simple DE....but lack of practice.
Hey,
I haven't looked at differential equations for some years... and I'm having trouble with two really simple ones....(Speechless)

$\frac{dx}{dt} = (x + 2), x(0) = 1$ ........ODE (1)

and $\frac{dy}{dt} = 2y, y(0) = 1$............ ODE (2)

The text I'm looking at says the answers for ODE (1) and ODE (2) should be $x + 2 = 3e^t$ and $y = e^{2t}$ respectively.

I thought the solutions (not imposing the initial conditions are) for each of the conditions are

$xt + 2t$ and $2yt$ respectively.

I must be doing this incorrectly. Do I use separation of variables?

$\int \frac{dx}{x+2} = \int \ dt$

and

$\int \frac{dy}{y} = \int 2 dt$ ?

If so, how do I integrate $\int \frac{dx}{x+2}$ and $\int \frac{dy}{y}$ ?

Thanks
• August 28th 2010, 06:33 AM
Sudharaka
Quote:

Originally Posted by JavaJunkie
Hey,
I haven't looked at differential equations for some years... and I'm having trouble with two really simple ones....(Speechless)

$\frac{dx}{dt} = (x + 2), x(0) = 1$ ........ODE (1)

and $\frac{dy}{dt} = 2y, y(0) = 1$............ ODE (2)

The text I'm looking at says the answers for ODE (1) and ODE (2) should be $x + 2 = 3e^t$ and $y = e^{2t}$ respectively.

I thought the solutions (not imposing the initial conditions are) for each of the conditions are

$xt + 2t$ and $2yt$ respectively.

I must be doing this incorrectly. Do I use separation of variables?

$\int \frac{dx}{x+2} = \int \ dt$

and

$\int \frac{dy}{y} = \int 2 dt$ ?

If so, how do I integrate $\int \frac{dx}{x+2}$ and $\int \frac{dy}{y}$ ?

Thanks

Dear JavaJunkie,

Yes you should use seperation of variables. And remember that, $\int{\frac{1}{x+c}}dx=\ln{\mid{x+c}\mid}~;~where~C ~is~an~arbitary~constant.$ Hope you would be able to continue.