
Isosceles Triangles Theorem
Presentation
•
Mathematics
•
9th - 12th Grade
•
Hard
Joseph Anderson
FREE Resource
12 Slides • 25 Questions
1
Use Isosceles and Equilateral Triangles
2
Show what you already know!
​
3
Multiple Choice
Classify the triangle by its sides: 2cm, 2cm, 2cm
equilateral
isosceles
scalene
4
Multiple Choice
Classify the triangle by its sides: 7ft, 11ft, 7ft
equilateral
isosceles
scalene
5
Multiple Choice
Classify the triangle by its sides: 9m, 8m, 10m
equilateral
isosceles
scalene
6
Math spoken here.
Vocabulary:
Legs: when an isosceles triangle has exactly two congruent sides, these sides are called legs.
Vertex angle: the angle formed by the legs.
Base: The third side of the isosceles triangle.
Base Angle: The two angles adjacent to the base.
7
Multiple Choice
Segment AB
Segment BC
Segment AC
8
Multiple Choice
<C
<A
<B
9
Theorems:
*Base Angles Theorem
*Converse of the Base Angles Theorem
10
Base Angles Theorem:
If two sides are congruent, then the angles opposite them are congruent.
11
Converse of the Base Angles Theorem:
If two angles are congruent, then the sides opposite them are congruent.
12
Multiple Choice
For isosceles triangles, when we are given that two sides are congruent we can prove that ______________________.
two angles are congruent
the third side is congruent
three angles are congruent
no other relationships exist
13
Multiple Select
In ΔDEF, DE ≈DF. Use the base angles theorem to choose the two congruent angles.
∠D
∠E
∠F
14
Multiple Choice
Complete the statement.
If HG≈HK, then ∠HGK ≈∠…
GHK
KGH
HKG
GKH
15
Multiple Choice
Complete the statement. If ∠KHJ≈∠KJH, then KH≈…
KH
KJ
JH
16
Finding Measures: Angles
Isosceles Triangles
17
18
Multiple Choice
Use the triangle sum theorem and the base angles theorem to find a.
45
50
80
25
19
Finding Measures: Legs
Isosceles Triangles
By the converse of the base angles theorem c = 14.
20
Multiple Choice
Find the measure of the missing side.
7
13
20
14.8
21
Corollaries
Recall:
Equilateral triangles are special cases of isosceles triangles that have three congruent sides.
22
Multiple Choice
Find m∠P.
60
80
45
90
23
Challenge!
Find the value of x and y.
24
Multiple Choice
Find the value of x and y.
x = 50, y = 1/2
x = 1/2, y = 50
x = 50, y = 2
x = 2, y = 50
25
Takeaways:
- If two sides of a triangle are congruent, then the angles opposite are also congruent. The converse is also true.
- If a triangle is equilateral, then it is also equiangular.
26
Multiple Choice
27
Multiple Choice
What is the measure of BC?
12
6
9
10
28
Multiple Choice
29
Multiple Choice
30
Multiple Choice
Solve for x.
72o
144o
46o
36o
31
Fill in the Blanks
Type answer...
32
Multiple Choice
Find the value of x. (Hint: What type of triangle is this?)
4
1
-4
10
33
Multiple Choice
Find the value of y.
60
30
20
12
34
Multiple Choice
Find the value of x.
30°
45°
60°
12°
35
Multiple Choice
Find the value of x. (Hint: what kind of triangle is this?)
43°
47°
94°
137°
36
Multiple Choice
Find the value of y.
5
10
15
20
37
Multiple Choice
Triangle KLM is equilateral with KM = x + 2, LM = 12 – x, and KL = 4x – 13.
Find the value of x.
(Hint: draw a picture)
2
3
4
5
Use Isosceles and Equilateral Triangles
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