Proving Theorems about Triangles
• Materials: device with internet, google account
• Prerequisite Skills:
There are many prerequisite justifications for this lesson. they are listed in the lesson to refresh the mind, but not defined or explained. Definitions include: parallel lines, perpendicular lines, complementary angles, supplementary angles, congruent angles, right angles, vertical angles, linear pairs
• Interior and exterior angles of a triangle are explicitly defined with visuals.
Isosceles triangles are defined.
• Base Angles of an Isosceles Triangle Theorem is shown, but will be proven when students study congruent triangles. The theorem is used as a justification, as well as solving for variables in more traditional problems.
• The triangle sum theorem and exterior angles of a triangle theorem are proven as part of this lesson. These proofs are scaffolded to help develop skills for writing proofs. A proof using base angles of an isosceles triangle theorem follows these proofs.
• A quick lesson summary helps students reflect upon the new theorems learned.
• 5 Practice proofs follow the lesson and may be used as an assignment or formative assessment
• The 6th practice problem is a set of 4 figures that ask students to solve for the variable. Transferring the meaning of the new theorems will be used to solve these 4 problems.
• If you would not like your students to have the answers to grade as they go, simply delete those slides before sharing the file with your students.
• Note: Lesson format is compatible with Google slides®
• Answer keys included