Author note — to cover

Derive the 180-degree angle sum with a parallel line through a vertex and alternate angles — G3 says derive, not just use — then apply it, including algebraic angles and triangles nested in other figures. The exterior-angle shortcut and the isosceles special case are sibling pages. Both tiers. (G3)

• Fluency — direct questions on the angle sum of a triangle, limited to this lesson's stated scope (4 questions).
• Application — use the same skill in unfamiliar or contextual questions without introducing a later method (4 questions).
• Check — questions built around the exclusions and likely misconceptions named in the lesson brief (4 questions).