Curriculum placement
Grade 8. This describes the skill, not a learner estimate.
Angle sums and exterior angles
Reason about triangle sums exterior angles and parallel lines
Problem types
- Angles of a triangle
- Exterior angle of a triangle
- Parallel lines and a transversal
- Angle chains with parallel lines and triangles
- Find the flaw in a triangle angle argument
- Reasons in the triangle angle arguments
- Why parallel lines make equal angles
- Reasons for angles at parallel lines
- Two angles are enough for similar triangles
- Which triangles must be similar
- Exterior angle of an isosceles triangle with algebraic base angles
Builds on
Leads to
- Writing and critiquing a geometric proof
- Theorems about circles · See it applied
Angle sums in triangles are used to find angles in circles, such as in isosceles triangles made of two radii.
- Regular polygons inscribed in a circle · See it applied
Angles of a polygon in a circle are found by splitting it into triangles whose angles sum to 180 degrees.
Related skills
- When given measures make a triangle · Related idea
Three angles can form a triangle only if they total 180 degrees, the angle sum theorem.
Open the interactive map to explore lessons and skill connections.