Chapter 4.1 Notes: Apply Triangle Sum Properties

1
Lessons 4.1 and 4.2 Triangle Sum Properties
Properties of Isosceles Triangles -Classify
triangles and find measures of their angles. -
Discover the properties of Isosceles Triangles.
HOMEWORK Lesson 4.1/1-9 and 4.2/1-10
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Warm-Up

• Name 2 pair of alternate interior angles
• lt5 lt3 and lt4 lt1
• 2. What is the sum of mlt1 mlt2 mlt3?
• 180
• If mlt4 65 and mlt5 50, what is mlt2?
• 65

4. Find all the angle measures
50
140
3
Classification By Sides
Classification By Angles
4
Classifying Triangles

• In classifying triangles, be as specific as
  possible.

Obtuse, Isosceles
Acute, Scalene
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Triangle Sum Theorem NEW

• The sum of the measures of the interior angles of
  a triangle is 180o.

mlt1 mlt2 mlt3 180
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Property of triangles

• The sum of all the angles
• equals 180º degrees.

60º
90º
30º

60º
180º
30º
90º
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Property of triangles

• The sum of all the angles
• equals 180º degrees.

40º
90º
40º
50º

180º
50º
90º
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Property of triangles

• The sum of all the angles
• equals 180º degrees.

60º
60º
60º
60º

180º
60º
60º
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What is the missing angle?
70º
70º
?
?

180º
70º
70º
180 140 40
10
What is the missing angle?
90º
?
30º
?

180º
90º
30º
180 120 60
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What is the missing angle?
?
60º
60º
?

60º
60º
180º
189 120 60
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What is the missing angle?
?
30º
78º
?

78º
30º
180º
180 108 72
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Find all the angle measures
180 35x 45x 10x
180 90x
2 x
90, 70, 20
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What can we find out?

• The ladder is leaning on the ground at a 75º
  angle. At what angle is the top of the ladder
  touching the building?

180 75 90 x
180 165 x
15 x
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Corollary to Triangle Sum Theorem

• A corollary is a statement that readily follows
  from a theorem.

The acute angles of a right triangle are
complementary.
m?A m?B 90o
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Find the missing angles.

• The tiled staircase shown below forms a right
  triangle.
• The measure of one acute angle in the triangle is
  twice the measure of the other angle.
• Find the measure of each acute angle.

Cont
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Find the missing angles.
SOLUTION
2x x 90
3x 90
x 30
2x 60
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Find the missing angles.
2x (x 6) 90
2x 2(32) 64
3x 6 90
3x 96
(x 6) 32 6 26
x 32
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Isosceles Triangle at least two sides have the
same length
5 m
5 m
5 m
9 in
9 in
3 miles
3 miles
4 miles
4 in
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Properties of an Isosceles Triangle

• Has at least 2 equal sides
• Has 2 equal angles
• Has 1 line of symmetry

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Parts of an Isosceles Triangle
The vertex angle is the angle between two
congruent sides
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Parts of an Isosceles Triangle
The base angles are the angles opposite the
congruent sides
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Parts of an Isosceles Triangle
The base is the side opposite the vertex angle
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Isosceles Triangle Conjecture If a triangle is
isosceles, then base angles are congruent.
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Converse of Isosceles Triangle Conjecture If a
triangle has two congruent angles, then it is an
isosceles triangle.
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Equilateral Triangle Triangle with all three
sides are congruent
7 ft
7 ft
7 ft
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Equilateral Triangle Conjecture An equilateral
triangle is equiangular, and an equiangular
triangle is equilateral.
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Find the missing angle measures.
lt68 and lt a are base angles ? they are congruent
 
b
m?a
68
Triangle sum to find ltb
mltb 180 68 - 68
mltb 180 -136
m?b
44
68
a
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Find the missing angle measures.
ltc ltd are base angles and are congruent
Triangle sum 180 180 119 c d 180 119
c d 61 c d
 
ltc ½ (61) 30.5 ltd ½ (61) 30.5
m?c m?d
30.5
119
30.5
c
d
30
Find the missing angle measures.
E
EFG is an equilateral triangle ltE ltF ltG 180
/3 60
 
m?E m?F m?G
60
60
G
F
60
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Find the missing angle measures.
Find m?G.
?GHJ is isosceles lt G lt J
 
x 44 3x 44 2x
x 22
Thus mltG 22 44 66 And mltJ 3(22) 66
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Find the missing angle measures.
Find m?N
Base angles are
6y 8y 16 -2y -16 y 8
Thus mltN 6(8) 48. mltP 8(8) 16 48
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Find the missing angle measures.
Using Properties of Equilateral Triangles
Find the value of x.
?LKM is equilateral mltK mltL mltM
 
180/3 60
2x 32 60 2x 37 x 18.5
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Find the missing side measures.
Using Properties of Equilateral Triangles
Find the value of y.
 
?NPO is equiangular ?NPO is also equilateral.
ft
ft
5y 6 4y 12 y 6 12 y 18
Side NO 5(18) 6 90ft
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Find the missing angle measures.
Using the symbols describing shapes answer the
following questions
Equilateral triangle all angles are equal
Isosceles triangle Two angles are equal
Right-angled triangle
c 180 3 60o
a 36o
d 180 (45 90) 45o
b 180 (2 36) 108o
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Find the missing angle measures.
Kite - Made up of 2 isosceles triangles
p 36o
q 180 (2 36) 108o
56 (r s) 180o
(r s) 180 56 124
Because r s
r s 124 2 62o
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Find the missing angle measures.
A
D
B
C
A
B
C
Equilateral triangle
e f g 60o
c d
a 64o
c d 180 - 72
b 180 (2 64o ) 52o
h i
D
c d 108
h i 180 - 90
c d 54o
h i 90
h i 45o
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p 50o
q 180 (2 50o ) 80o
r q 80o
vertical angles are equal
Therefore s t p 50o
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Find the missing angle measures.
Properties of Triangles
p q r 60o
a b c 60o
d 180 60 120o
s t 180 - 43 68.5o 2
e 18 a 60
exterior angle sum of remote interior angles
e 60 18 42o
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Find the missing angle measures.

1. Find the value of x
2. Find the value of y

• x is a base angle
• 180 x x 50
• 130 2x
• x 65

2) y z are remote interior angles and base
angles of an isosceles triangle Therefore y z
x and y z y z 80 y 40
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Find the missing angle measures.

1. Find the value of x
2. Find the value of y

1) ?CDE is equilateral All angles 60 Using
Linear Pair ltBCD 70 x is the vertex angle x
180 70 70 x 40
70
60
2) y is the vertex angle
y 180 100 y 80
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Homework

• In your textbook
• Lesson 4.1/ 1-9 4.2/ 1-10