lesson 7 2 - alonzo and tracy mourning senior high ...€¦ · 15/08/2019 · triangle, so the 2...
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Lesson 7.2
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Isosceles Triangles Fundamentals
• There are two congruent sides in an isosceles triangle and two congruent angles.
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Vertex Angle
Base
Base Angles
Legs
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Equilateral Triangle Fundamentals
• All of the sides and angles are congruent. • The angles in an equilateral triangle always equal 60o.
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� If two sides of a triangle are congruent, then the base angles are also congruent.
� Base angles are the angles at the ends of the 2 congruent segments.
� So, in the diagram angles B and C are congruent.
Base angle Base angle
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Isosceles Triangle Theorem (AKA Base Angles Theorem)
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• If two angles in a triangle are congruent, then the triangle is an isosceles triangle.
• That means that the 2 sides of the triangles are also congruent.
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Converse to the Isosceles Triangle Theorem
(AKA Base Angles Theorem)
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Example 1: Find the value of x.
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This is an isosceles triangle, so the 2 sides are congruent…
1.
2. 3.
Example 2: Find the value of x and y.
5x + 5 = 35 5x = 30 x = 6
x + x + 102 = 180 2x + 102 = 180
2x = 78 x = 39 y = 39
55 + 55 + y = 180 110 + y = 180
y = 70 x + 7 = 55
x = 48
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Example 3: Find the value of x.
x + x + 3x = 180 5x = 180
x = 36
x°
x
∠𝑇=3𝑥 ∠𝑇=3(36)
∠𝑇=108
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Theorem
Converse
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Equilateral Triangle Theorem Equiangular Triangle Theorem
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C Isosceles Triangle Theorem
AC ≅ BC
Substitution
Transitive
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AC
Converse of the Isosceles Triangle Theorem
AB ≅ BC
∠A
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Example 3A: Using Properties of Equilateral Triangles
Find the value of x.
∆LKM is equilateral.
(2x + 32)° = 60° The measure of each ∠ of an equiangular ∆ is 60°.
2x = 28 Subtract 32 both sides.
x = 14 Divide both sides by 2.
Equilateral ∆ à equiangular ∆
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Example 3B: Using Properties of Equilateral Triangles
Find the value of y. ∆NPO is equiangular. Equiangular ∆ à equilateral ∆
5y – 6 = 4y + 12 Definition of equilateral ∆.
y = 18 Subtract 4y and add 6 to both sides.
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