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Things to remember about Euler's Method:
The error in an approximation is the difference between the approximation and the real value of the function.
We can only figure out the error if we know the real value of the function.
Our first examples of Euler's method used the function y that satisfied the IVP
By thinking backwards we can figure out that
y = x2.
Now we can compute the real value of y at 1:
y(1) = (1)2 = 1.
This lets us figure out how bad our approximations were in some earlier examples.
Using a step size of 0.5 we estimated
y(1) ≈ 0.5.
This has an error of 0.5, since it's 0.5 off from the real answer y(1) = 1.
Using a step size of 0.25 we estimated
y(1) ≈ 0.75.
The error here is 0.25.
Using a step size of 0.2 we estimated
y(1) ≈ 0.8.
This has an error of 0.2.