- #1

anb

- 3

- 0

## Homework Statement

So for the question we have (dx/dt) = f(x) = -x with initial condition x(0) = 1. The first part asks us to give an exact solution to x(1). I believe the solution is (e^-1) and I'm not having trouble with this part (but if that's incorrect then maybe that's my problem - I reasoned that you could just separate variables and integrate both sides, giving -lnx=t+K for some constant K. x=1 at t=0 so K=0 since ln1=0. So then if t=1 and K=0, lnx=-1 which means x=e^-1).

The second part says "using Euler's method with step size D=1 [delta was used in the text but I can't seem to get a "Delta" symbol working], estimate x(1) numerically ... then repeat using D=(10^-n) for n=1,2,3..."

## Homework Equations

## The Attempt at a Solution

Now here's what I did. The text tells us that to approximate solutions with euler's method we use the recursion x

_{n+1}=x

_{n}+f(x

_{n})*D. So I take x

_{0}=1 since x(0)=1 is the initial condition. Therefore f(x

_{0})=-1 since f(x)=-x, and with D=t that gives us the approximation x

_{1}=0 (because this is such a poor approximation I assume I may have gone wrong here, ie maybe I just don't understand which values to plug where).

Now when it asks us to repeat with different step sizes D=(10^-n), I'm not sure if it means that for approximating x

_{n+1}we should use D=(10^-n), or if the text is asking us to repeat using that step size for the approximation of x

_{1}. The latter doesn't really make sense to me, because that would lead to an even worse approximation for all n>0. My problem though is that if I repeat the incursion for n>0, I'm pretty sure I get

x

_{n+1}=0 for all n>1 since the approximation x

_{1}=0, so x

_{2}=x

_{1}+f(x

_{1})*D=0 no matter what D is. I realize that this could still be the intended way of solving the problem, it just doesn't seem very nice because the actual step size doesn't matter and I'm used to having to use every part of a problem to solve it (and in this case the step size seems like a red herring).

Keep in mind I've never worked with euler's method before (shock), and I may just be over-thinking the problem