Practice this lesson yourself on KhanAcademy.org right now:

https://www.khanacademy.org/math/differential-calculus/taking-derivatives/derivative_intro/e/the-formal-and-alternate-form-of-the-derivative?utm_source=YT&utm_medium=Desc&utm_campaign=DifferentialCalculus

Missed the previous lesson?

https://www.khanacademy.org/math/differential-calculus/taking-derivatives/derivative_intro/v/calculus-derivatives-2-5-new-hd-version?utm_source=YT&utm_medium=Desc&utm_campaign=DifferentialCalculus

Differential calculus on Khan Academy: Limit introduction, squeeze theorem, and epsilon-delta definition of limits.

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