Download Notes Section 4-2

Geometry
Notes Section 4-2
What you’ll learn. . . .
How to apply the angle sum
theorem
 How to recognize exterior angles
and apply the exterior angle
theorem

Vocabulary



Exterior angle of a triangle
Remote interior angles
Third angle theorem
The Angle Sum Theorem
Consider the sum of the angles
of a triangle. . . .

The sum of the measures of the angles
of a triangle will always be 180°
Example #1:Determine the
angle measures.
Example #2:Determine the
angle measures.
Example #3:Determine the
angle measures
Example #4:Determine the
angle measure
The Exterior Angle Theorem
2
4
1
3
Exterior Angles will always make a
linear pair with an interior angle of a
triangle.

Each exterior angle will be associated with 2
remote interior angles (the two angles not
connected to that exterior angle).
The exterior angle theorem:
Exterior angles and their
remote interior angles share a
special relationship.

The measure of an exterior angle of a triangle
will always be equal to the sum of the measures
of its remote interior angles.
Example Problem #5:

Find x and y.
The Third angle Theorem
5
4
6
2
1
3
If two angles of one triangle measure
the same as two angles from another
triangle, their third angles must also be
the same measure.
Example Problem #6:

Find x and y.
Have you learned. . . .
How to apply the angle sum
theorem
 How to recognize exterior angles
and apply the exterior angle
theorem


Assignment:


Non-Proof: Worksheet 4-2
Proof: p.189 (15-38); all diagrams
must be drawn in your homework