Geometry/Chapter 4/Lesson 1

Introduction

What is a triangle?

A triangle is a figure formed by 3 segments joining 3 noncollinear points. Each of these 3 points [in the triangle] are a vertex of the triangle. The segments are what is known as the sides of the triangle.

How do we classify triangles?

We can classify them by their angles or sides. This lesson we will go over how to classify triangles by their angles. Next lesson (Geometry/Chapter 4/Lesson 2), you will go over the 2nd way of classifying triangles: By their sides.

Classifying Triangles by Angles

One way of classifying triangles is by their angles. All triangles have at least 2 acute angles, but the third angle is used to classify what type of triangle it is.

Acute Triangle

In an acute triangle, ALL of the angles are acute (<90). There is no single angle in this triangle that is (≥ 90).

Obtuse Triangle

In an obtuse triangle, only ONE angle is obtuse. Only one angle in this triangle is (> 90).

Right Triangle

In a right triangle, only ONE angle is right (= 90).

Equiangular triangle

In an equiangular triangle, ALL angles are congruent (<1 = <2, <2 = <3, <1 = <3).