Lesson 10-4 Pages 428-431

Triangles

What you will learn!

How to identify and classify triangles.

TriangleTriangle Congruent segmentsCongruent segments

Acute triangleAcute triangle Scalene triangleScalene triangle

Right triangleRight triangle Isosceles triangleIsosceles triangle

Obtuse triangleObtuse triangle Equilateral triangleEquilateral triangle

What you really need to know!

A triangle is a figure with three sides and three angles. The sum of the measures of the angles of a triangle is 180. You can use this to find a missing angle measure in a triangle.

Triangles can be classified bythe measures of their angles:

1. An acute triangle has three acute angles.

2. An obtuse triangle has one obtuse angle.

3. A right triangle has one right angle.

Triangles can also be classifiedby the lengths of their sides:

1. In a scalene triangle, all sides have different lengths.

2. An isosceles triangle has at least two congruent sides.

3. An equilateral triangle has all three sides congruent.

What you really need to know!

Sides that are the same length are congruent segments and are often marked by tick marks.

Example 1:

An airplane has wings that are shaped like triangles. Find the missing measure.

112 + 47 = 159

180 – 159 = 21

21°

Example 2:

Find m A in ∆ABC if m A = m B and m C = 80°.

180 = m A + m B + m C

180 = m A + m B + m C

180 = m A + m B + 80

100 = 50 + 50100 = m A + m B

m A = 50° m B = 50°

Example 3:

Classify the triangle by its angles and by its sides.

Obtuse Isosceles30° 30°

120°

Example 4:

Classify the triangle by its angles and by its sides.

Acute Scalene

80°

70°

30°

Page 430

Guided Practice

#’s 3-7

Pages 428-429 with someone at home and study

examples!

Read:

Homework: Page 430-431

#’s 8-24 all

#’s 27-34 all

Lesson Check 10-4

Page

588

Lesson 10-4