Isosoleces Triangle

By definition, an isosceles triangle is a triangle that has at least two congruent sides. This means that an equilateral triangle is an isosceles triangle.

Isosceles Triangle

Figure 1 Isosceles Triangle

\Delta{ABC} in figure 1 is an isosceles triangle because \overline{AB} \cong \overline{AC}. This would be by the definition of an isosceles triangle. There are other ways to justify a triangle is an isosceles triangle besides the definition. I talk about that in another post. I want to focus in on the parts of an isosceles triangle in this post.

For any isosceles triangle, the legs are congruent and form the vertex angle. The vertex angle is opposite the 3rd side of the triangle. The 3rd side is called the base and helps form the base angles. The base angles are opposite the legs and the base angles are congruent. For \Delta{ABC} in figure 1, \angle{A} is the vertex angle, \angle{B} \cong \angle{C} because they are base angles,  \overline{AB} \cong \overline{AC} because they are the legs and \overline{BC} is the base.



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