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Lesson 4-2
Angles of Triangles
Ohio Content Standards:
Ohio Content Standards:
Establish the validity of conjectures about geometric objects, their properties and relationships by counter-example, inductive and
deductive reasoning, and critiquing arguments made by others.
Ohio Content Standards:
Make and test conjectures about characteristics and properties (e.g.,
sides, angles, symmetry) of two-dimensional figures and three-
dimensional objects.
Theorem 4.1Angle Sum Theorem
Theorem 4.1Angle Sum Theorem
The sum of the measures of the angles of a triangle is 180.
Find the missing angle measures.
43°
3
21
74°
79°
Theorem 4.2Third Angle Theorem
Theorem 4.2Third Angle Theorem
If two angles of one triangle are congruent to two angles of a
second triangle, then the third angles of the triangles are
congruent.
Exterior Angle
Exterior Angle
Angle formed by one side of a triangle and the extension of
another side.
Exterior Angle
Angle formed by one side of a triangle and the extension of
another side.
1
Remote Interior Angles
Remote Interior Angles
Interior angles of the triangle not adjacent to a given exterior angle.
Remote Interior Angles
Interior angles of the triangle not adjacent to a given exterior angle.
2
3
Theorem 4.3Exterior Angle Theorem
Theorem 4.3Exterior Angle Theorem
The measure of an exterior angle of a triangle is equal to the sum of