18.6.5 A Cone's Side and Its Unrolling
Core ideaCut the side of a cone open and flatten it out and you get a sector, whose arc length is exactly the circumference of the base circle.
Module goal. Move from the circumference being pi times the diameter to arc length, sector area, and regular polygons, gathering geometry into quantities you can compute, and paving the way for the next stage's use of numbers and intervals to describe shapes.