PJYelton

08-19-2003, 09:31 AM

I just started a new class that has Differential Equations as a prerequisite, so as a refresher our teacher gave us a little assignment (not for credit so asking this wouldn't be cheating)that had many calc and DE's refresher questions. But its been a few years since I took the DE's class and one of the questions is screwing me up... anyone know if I am doing this right?

The question: dx/dt + 3x = 0, x(0)=2, x(t)=?

Okay, if memory serves, the first thing to do is get the dx and dt on separate sides and then integrate.

dx/dt=-3x

dt=(-1/3)dx

Integrating... t=(-1/3) ln(x) + C

Plugging in the values above to solve for C... 2=(-1/3) ln (0) + C

But the natural log of zero is undefined, so I know that I am doing something wrong... any help? :confused:

The question: dx/dt + 3x = 0, x(0)=2, x(t)=?

Okay, if memory serves, the first thing to do is get the dx and dt on separate sides and then integrate.

dx/dt=-3x

dt=(-1/3)dx

Integrating... t=(-1/3) ln(x) + C

Plugging in the values above to solve for C... 2=(-1/3) ln (0) + C

But the natural log of zero is undefined, so I know that I am doing something wrong... any help? :confused: