Dividing Polygons into Triangles (Part II)

Dividing Polygons into Triangles (Part II) extends the Grade 8 geometry investigation from specific polygons to a general interior-angle formula. Students examine hexagons, octagons, decagons, and other polygons, compare the number of sides with the number of non-overlapping triangles, and look for the relationship that leads to the angle-sum rule. The pattern gives the formula a geometric reason.

Use it as a whole-class follow-up after students have already decomposed simpler polygons. Have students justify the number of triangles before substituting into a formula so the structure stays meaningful. Create a free Find a Math Game account to generate the lesson for Dividing Polygons into Triangles (Part II). No credit card; 3 lifetime generations.