18.6.3 The Arc-Length Formula
Core ideaWhatever fraction of the full circle an arc takes up, its length is that same fraction of the whole circumference 2 pi r — that is, l = n pi r ÷ 180.
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.