Section 3-3 Parallel Lines and the Triangle Angle-Sum Theorem

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Section 3-3Parallel Lines and the Triangle
Angle-Sum Theorem
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Activity 1
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2
3
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Activity 2
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2
3
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Formal Proof
A
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Triangle Angle-Sum Theorem The sum of the
measures of the angles of a triangle is 180.
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5
B
C
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Formal Proof
A
Triangle Exterior Angle Theorem The measure of
each exterior angle of a triangle equals the sum
of the measures of its two remote interior angles.
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D
4
1
3
B
C
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Classifying Triangles
Classify by Angles
60
60
60
Obtuse
Acute
Equiangular
Right
Classify by Sides
Equilateral
Isosceles
Scalene
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Example 1
x
67
48
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Example 2
z
70
x
y
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Example 3classify by angles and sides
5
2
120
4
10
Example 4
125
X
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Example 5
A triangle with a 90 angle has sides that are 3
cm, 4 cm, and 5 cm long. Classify the triangle
by its angles and sides.
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Example 6
y
70
42
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Example 7
90
76
x
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Example 8
2x 28
32
4x
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Example 9
5x 40
10x
3x - 4
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Example 10
x
160
125