Second video in a three-part explanation of the chain rule. Part 1 is the intro and informal explanation with two simple examples. Part 2 has a flawed "proof" and covers an extended version of the chain rule (with more than two functions). Part 3 is the formal proof with a detailed explanation. Part 3 was actually made before parts 1 and 2 and can be viewed on its own, but it will be easier to understand if parts 1 and 2 are viewed first.