# Homework Help: Sauce's and kine's

1. Feb 14, 2006

### jamesbob

$$iii. \frac{dx}{dt} + x - -3sin2t + 4cos2t$$

Can anyone help with this. All i know is to set

$$x = a\cos2t + b\sin2t$$

I dont even know if that is right

2. Feb 14, 2006

### TD

Is this a differential equation? I'm missing the equation part, in that case. Do you mean the following?

$$\frac{dx}{dt} + x = -3\sin(2t) + 4\cos(2t)$$

Last edited: Feb 14, 2006
3. Feb 14, 2006

### jamesbob

oh sorry, yeah thats what i meant to write

4. Feb 14, 2006

### TD

In that case, your proposal was fine for a particular solution but you need to add the solution of the associated homogeonous equation to obtain the complete solution. Luckily, that isn't too hard since what is the solution of the followin?

$$\frac{{dx}}{{dt}} + x = 0 \Leftrightarrow \frac{{dx}}{{dt}} = - x$$

If you don't see it immediately, integrate.

To find the coefficients a and b of your particular solution for x, find dx/dt and substitute in the equation to identify coefficients.