Prove theorems about triangles
Teacher tools for this standard
Lesson Plan · Guided Notes · Exit Ticket · Re-teach · Homework
Teacher tools for this standard
Lesson Plan · Guided Notes · Exit Ticket · Re-teach · Homework
- Lesson Plan →Objectives, pacing and practice, built from this lesson's brief.
- Guided Notes →One page your students fill in and keep.
- Exit Ticket →Three items at the end of class. No student accounts.
- Re-teach →After an exit ticket: who missed what, and what to do tomorrow.
- Homework →Assign practice; it grades itself.
**HSG.CO.C.10**: Prove theorems about triangles. Theorems include: measures of interior angles of a triangle sum to 180 degrees; base angles of isosceles triangles are congruent; the segment joining midpoints of two sides of a triangle is parallel to the third side and half the length; the medians of a triangle meet at a point.
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HSG.CO.C.10: Prove theorems about triangles. Theorems include: measures of interior angles of a triangle sum to 180 degrees; base angles of isosceles triangles are congruent; the segment joining midpoints of two sides of a triangle is parallel to the third side and half the length; the medians of a triangle meet at a point.
What you'll learn
- Prove that the measures of the interior angles of a triangle sum to 180 degrees, using a construction involving a line parallel to one side through the opposite vertex
- Prove that the base angles of an isosceles triangle are congruent, using either triangle congruence criteria or rigid motions (reflection)
- Prove the Triangle Midsegment Theorem: the segment joining the midpoints of two sides of a triangle is parallel to the third side and half its length
- Explain why the three medians of a triangle are concurrent and identify the centroid as the point that divides each median in a 2:1 ratio from vertex to midpoint
- Apply these triangle theorems to solve geometric problems and construct further proofs
Slides
Step through the lesson, or watch it as a narrated video • 2 slide decks
Angle and congruence theorems
✓ Start hereMidsegment and centroid
Practice
Try it on your own