16.1.3 The Interior-Angle Sum of a Polygon
Core ideaDraw diagonals from one vertex to slice it into triangles, an n-gon splits into (n-2) of them, so the interior angles add up to (n-2)·180°
Module goal. Take the triangles you just learned, join and generalize them into polygons, and master the language of polygons along with the two core relationships: interior-angle sum and exterior-angle sum