Section 1.7-Formal Definition of the Limit
The formal definition of the limit is probably the most abstract (and hence most difficult) concept we will discuss in Calculus I. I found an interactive diagram at http://www.scottsarra.org/applets/calculus/EpsilonDelta.html that does this well. It is part of a larger collection: http://www.scottsarra.org/applets/applets.html . I also produced one myself with GeoGebra that you find below.