- #1
perishingtardi
- 21
- 1
Suppose I have a variable separable ODE, e.g.,
[tex] \frac{dy}{dx} = 3y. [/tex]
We all know that the solution is [itex]y=Ae^{3x}[/itex] where A is a constant. My question is as follows. To actually find this solution we rearrange the equation and integrate to get
[tex] \int \frac{dy}{y} = 3 \int dx, [/tex]
which gives
[tex] \ln |y| = 3x + C [/tex] where C is a constant. I would have thought that this gives the solution
[tex] |y| = Ae^{3x} \qquad \mbox{where} \qquad A=e^C. [/tex]
My question is how can we get rid of the absolute value sign in the actual answer? Is it because A = e^C must always be positive? But how come in general that does not have to be true for the ODE to be satisfied?
[tex] \frac{dy}{dx} = 3y. [/tex]
We all know that the solution is [itex]y=Ae^{3x}[/itex] where A is a constant. My question is as follows. To actually find this solution we rearrange the equation and integrate to get
[tex] \int \frac{dy}{y} = 3 \int dx, [/tex]
which gives
[tex] \ln |y| = 3x + C [/tex] where C is a constant. I would have thought that this gives the solution
[tex] |y| = Ae^{3x} \qquad \mbox{where} \qquad A=e^C. [/tex]
My question is how can we get rid of the absolute value sign in the actual answer? Is it because A = e^C must always be positive? But how come in general that does not have to be true for the ODE to be satisfied?