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Isosceles Triangles

Isosceles Triangles

Author: Leif Park Jordan

    Define the different parts of an isosceles triangle (legs, base, base angles).


    Introduce the Base Angles Theorem (if two sides of a triangle are congruent, then the angles opposite them are congruent).


    Introduce the converse to the Base Angles Theorem (if two angles of a triangle are congruent, then the sides opposite them are congruent).


    Provide examples that demonstrate how to use these theorems to solve for unknown variables and unknown leg lengths and angle measurements.


This packet should help a learner seeking to understand the relationship between the base angles of an isosceles triangle and its side lengths.

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Introduction to Isosceles Triangles

This video introduces isosceles triangles.

Source: FernandoGuerra on Guaranteach


Legs: The legs of an isosceles triangle are the two sides that are congruent (equal in length). In the triangle  below, sides AB and BC are the legs.

Base: The base of a triangle is the side that is not congruent to the other two sides. In the case of an equilateral triangle, the choice of which side is the base is essentially arbitrary - any side could be considered the base. In the triangle below, side AC is the leg.

Base Angles: The base angles of a triangle are the two angles that have the base of the triangle as one of their sides (and have their vertex at one of the endpoints of the base). They are of equal measure. In the triangle below, angles A and C are the base angles.

Isosceles Triangle

Base Angles Theorem

This video introduces the base angles theorem and its converse.

Practice Problems

This slideshow presents a couple of examples that learners can use to test their understanding of the material in this packet.