4.6Isosceles Triangles
Quick Answer
Isosceles Triangle Theorem: if two sides of a triangle are congruent, the angles opposite those sides are congruent (base angles are equal). The converse is also true: if two angles are congruent, the opposite sides are congruent.
Essential Question
What special angle relationships exist in an isosceles triangle, and how do we use them to solve problems?
Learning Goals
- 1Identify the legs, base, and base angles of an isosceles triangle.
- 2Apply the Isosceles Triangle Theorem.
- 3Apply the converse of the Isosceles Triangle Theorem.
- 4Find missing side lengths and angle measures in isosceles triangles.
Key Vocabulary
Legs (isosceles)
The two congruent sides of an isosceles triangle.
Base
The non-congruent side of an isosceles triangle.
Base angles
The two angles at the ends of the base; they are congruent.
Isosceles Triangle Theorem
If two sides of a triangle are congruent, then the angles opposite those sides are congruent.
Converse of Isosceles Triangle Theorem
If two angles of a triangle are congruent, then the sides opposite those angles are congruent.
Full lesson content coming soon
Worked examples, guided practice problems, a workbook, quiz, and downloadable PDF resources will be added to this lesson in Phase 2. The learning goals and vocabulary above reflect the complete lesson scope.