4.3 Isosceles and Equilateral Triangles 185
Goal
Use properties of isosceles
and equilateral triangles.
Key Words
• legs of an isosceles
triangle
• base of an isosceles
triangle
• base angles
4.3
4.3
Isosceles and Equilateral
Triangles
The Geo-Activity shows that two angles of an
isosceles triangle are always congruent. These
angles are opposite the congruent sides.
The congruent sides of an isosceles triangle
are called .
The other side is called the .
The two angles at the base of the triangle
are called the .base angles
base
legs
●
3 Repeat Steps 1 and 2 for different isosceles triangles. What can
you say about a
H and aK in the different triangles?
●
1 Fold a sheet of paper in half.
●
2 Unfold and label the angles
Use a straightedge to draw a as shown. Use a protractor
line from the fold to the bottom to measure a
H and aK.
edge. Cut along the line to form What do you notice?
an isosceles triangle.
Properties of Isosceles Triangles
Geo-Activity
Base Angles Theorem
Words If two sides of a triangle are
congruent, then the angles opposite
them are congruent.
Symbols If AB
&*
c
AC&*, then aC c aB.