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Isosceles Triangles
Geometry
Ms. Reed
Unit 4, Day 2
Parts of an Isosceles Triangle
Legs
Base
Parts of an Isosceles Triangle
Vertex
Angle
Base Angles
With your Isosceles Triangle:
• Measure all three sides and angles
• With a partner, discuss your results.
• What did you notice?
Three Triangle Postulates
4-1: If two sides of a triangle are congruent,
then the angles opposite those sides are
also congruent.
What it looks like:
http://www.usna.edu/Users/cs/lmcdowel/courses/ic210/F07/labs/IC210_Lab_3_IsoscelesTriangle.gif
How to use it:
Find x.
http://education.yahoo.com/homework_help/math_help/solutionimages/minigeogt/4/1/1/minigeogt_4_1_1_19_30/f-226-31-ex-1.gif
Three Triangle Postulates
4-1: If two sides of a triangle are congruent,
then the angles opposite those sides are
also congruent.
4-2: The bisector of the vertex angle of an
isosceles triangle is the perpendicular
bisector of the base.
What it looks like:
CD is the bisector of
ACB. Make the
appropriate markings
on the triangle using
Theorem 4-2.
How to use it:
Find the perimeter and area of the triangle.
Three Triangle Postulates
4-1: If two sides of a triangle are congruent, then
the angles opposite those sides are also
congruent.
4-2: The bisector of the vertex angle of an isosceles
triangle is the perpendicular bisector of the
base.
4-3: If two angles of a triangle are congruent, then
the sides opposite the angles are congruent.
What is looks like:
How to use it:
• A  B, BC = 5. Find AC.
C
5
A
B
Practice
• Find the values of x
and y.
Homework
Work Packet:
Isosceles Triangles