Worksheet 8, Clip 2, Page 2 Discussion, Part II, 1:11
By having all three graphs on the board, the teacher is able to give a visual for the students as he discusses period.
He also points out that by "doubling the rate" you "halve the period". This is something that, due to the explorations the students have been doing, gives them a way to think about how changing the argument affects the period. This is opposed to the traditional method of having them memorize a rule.