In this clip the teacher leads a discussion about why arccos and arcsin gave different results.
Worksheet 6, Clip 2, Discussion of Question 2, 2:35
In order to get the students to realize what to dowhene radius is not 1, he reminds them of what units they need to be in. Students realize that they need to input the percentage of the radius so in order to do that the value must be divided by the radius.
To help reinforce this concept, the teacher asks about what the units will be. This creates a nice discussion about what the inverse trig functions give as an output and how those outputs can be used to get arclength.