Euler's Method Error: Derivation & Estimation

[Moose352]

I understand how the (local) error for euler's method of integration is derived from the perspective of the taylor expansion and inequality. However, I don't really see why taylor's equation needs to be invoked, since the euler method can also be derived as a tangent line approximation. How then is the order of the error estimated by interpreting euler's method as a tangent line approximation?

 

[Integral]

It is really the same thing. Using a Taylor series expansion, for the Euler method you truncate all but the linear terms, thus your approximation is assuming a local linear function. This is the exact same thing as using a tangent line approximation. You are assuming that the function is locally linear. When you say you are just making a tangent line approximation you are simply ignoring the fact that you are in reality just dropping the nonlinear terms of the Taylor series.