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4.1 Triangles and Angles. Geometry Mrs. Spitz Fall 2004. Standard/Objectives:. Standard 3: Students will learn and apply geometric concepts. Objectives: Classify triangles by their sides and angles. Find angle measures in triangles

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4 1 triangles and angles

4.1 Triangles and Angles


Mrs. Spitz

Fall 2004

Standard objectives

Standard 3: Students will learn and apply geometric concepts.


  • Classify triangles by their sides and angles.

  • Find angle measures in triangles

    DEFINITION: A triangle is a figure formed by three segments joining three non-collinear points.

4 1 homework
4.1 Homework

  • 4.1 Worksheet A and B

  • Chapter 4 Definitions – pg. 192

  • Chapter 4 Postulates/Theorems – green boxes within chapter 4

  • Binder check Monday/Tuesday

Names of triangles
Names of triangles

Triangles can be classified by the sides or by the angle

Equilateral—3 congruent sides

Isosceles Triangle—2 congruent sides

Scalene—no congruent sides

Acute triangle
Acute Triangle

3 acute angles

Equiangular triangle
Equiangular triangle

  • 3 congruent angles. An equiangular triangle is also acute.

Right triangle

1 right angle

Right Triangle

Obtuse Triangle

Parts of a triangle

Each of the three points joining the sides of a triangle is a vertex.(plural: vertices). A, B and C are vertices.

Two sides sharing a common vertext are adjacent sides.

The third is the side opposite an angle

Parts of a triangle


Side opposite A


Right triangle1

Red represents the hypotenuse of a right triangle. The sides that form the right angle are the legs.

Right Triangle




An isosceles triangle can have 3 congruent sides in which case it is equilateral. When an isosceles triangle has only two congruent sides, then these two sides are the legs of the isosceles triangle. The third is thebase.

Isosceles Triangles




Identifying the parts of an isosceles triangle

Explain why case it is ∆ABC is an isosceles right triangle.

In the diagram you are given that C is a right angle. By definition, then ∆ABC is a right triangle. Because AC = 5 ft and BC = 5 ft; AC BC. By definition, ∆ABC is also an isosceles triangle.

Identifying the parts of an isosceles triangle

About 7 ft.

5 ft

5 ft

Identifying the parts of an isosceles triangle1

Identify the legs and the hypotenuse of case it is ∆ABC. Which side is the base of the triangle?

Sides AC and BC are adjacent to the right angle, so they are the legs. Side AB is opposite the right angle, so it is t he hypotenuse. Because AC BC, side AB is also the base.

Identifying the parts of an isosceles triangle

Hypotenuse & Base

About 7 ft.

5 ft

5 ft



Using angle measures of triangles
Using Angle Measures of Triangles case it is

Smiley faces are interior angles and hearts represent the exterior angles

Each vertex has a pair of congruent exterior angles; however it is common to show only one exterior angle at each vertex.

Ex 3 finding an angle measure
Ex. 3 Finding an Angle Measure. case it is

Exterior Angle theorem: m1 = m A +m 1

x + 65 = (2x + 10)

65 = x +10

55 = x




Finding angle measures

Corollary to the triangle sum theorem case it is

The acute angles of a right triangle are complementary.

m A + m B = 90

Finding angle measures



Finding angle measures1

X + 2x = 90 case it is

3x = 90

X = 30

So m A = 30 and the m B=60

Finding angle measures