GeometrySection 4.1

Classifying Triangles

Triangle – 3-sided polygon named using its 3 vertices

Triangles are classified in two ways: sides and angles.

Angle ClassificationsAll triangles have at least 2 acute

angles, the third is used to classify the angle.

Acute triangle - 3 acute anglesEquiangular triangle – 3 congruent acute anglesObtuse triangle - 1 obtuse angleRight triangle - 1 right angle

Example 1Classify each triangle as acute,

equiangular, obtuse, or right.

A.

D.

C.B.

101°

60°

60°

60°

90° 70° 70°

40°

Side ClassificationsTriangles are also classified by the

number of congruent sides they have.

Equilateral triangle - 3 congruent sides

Isosceles triangle – at least 2 congruent sides

Scalene triangle – no congruent sides

Example 2Classify each triangle equilateral,

isosceles, or scalene.

60°

60°

60°8 in8 in

4 in

6 ft

8 ft

10 ft

Isosceles Triangles

Leg Leg

Base

Example 3

4x - 13 12 - x

x + 6K

M

L

Find the measures of the 3 sides of isosceles triangle KLM with base KL.

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