6.5: Classifying Triangles
Have you ever thought of making a tipi? Take a look at this dilemma.
Jaime has been working hard on her tipi. She has decided to use the following pattern as part of the design on the material. She thinks that if she uses cloth triangles, that she can sew them onto the cloth of the tipi to make a pattern. This red, black, red pattern is going to stretch along the outside bottom edge of the tipi.
“That is very cool,” her sister Lily said admiring Jaime’s sketch.
“I think so too. It should look very pretty on the brown material,” Jaime added.
“Yes. How do you know that the triangles will be the exact same size?”
“That’s easy. I can measure the top angle of the triangle using a protractor and then create each one by tracing. This one will have an angle of \begin{align*}120^{\circ}\end{align*}
“What kind of triangle is that?” Lily asked.
Do you know? This Concept is all about different types of triangles. As you learn all about the different types of triangles, think about this problem. What type of triangle is in the pattern? Can you justify your answer?
Guidance
In this Concept, we will examine different kinds of triangles. As we know, triangles are geometric figures that have three sides and three angles. There are different types of triangles too. We can classify or identify them in different ways. One way is by their angles measures and one way is by their sides lengths.
Let’s start with angle measures first.
We all know that triangles have three angles. The corners where each of the line segments connects form an angle. When we know the measures of these angles, we can use this information to name and identify the triangles.
Let’s look at some of the different types of triangles according to angle measure.
Acute Triangles are triangles that have three angles that are less than \begin{align*}90^{\circ}\end{align*}
Right Triangle a right triangle is a triangle with one \begin{align*}90^{\circ}\end{align*}
Obtuse Triangle is a triangle with one angle that is obtuse or greater than \begin{align*}90^{\circ}\end{align*}
Now let's apply this information.
Identify whether each of the following triangles is acute, obtuse or right.
Now let’s break each one down.
With the first triangle, triangle \begin{align*}a\end{align*}
The second triangle has three angles that are less than 90 degrees, this is an acute triangle.
The third triangle also has three angles that are less than 90 degrees. This is also an acute triangle.
The fourth triangle has a right angle. You can see that because it forms a nice neat corner so perfectly. This is a right triangle.
The fifth triangle has an angle greater than 90 degrees. This is an obtuse triangle.
We can also classify or identify triangles by the length of their sides. This means that we look at the line segments that create the triangle.
Equilateral Triangle is a triangle with all three sides equal.
Isosceles Triangle has two sides that are equal in length. Often an isosceles triangle is the trickiest one to identify.
Scalene Triangle is a triangle where none of the sides are the same length. All three sides are different lengths.
Now let’s apply what we have learned and identify some triangles.
Classify each triangle as equilateral, isosceles, or scalene.
We need to examine the lengths of the sides in each triangle to see if any sides are congruent.
In the first triangle, two sides are 7 meters long, but the third side is shorter. Which kind of triangle has two congruent sides? This is an isosceles triangle.
Now let’s look at the second triangle. All three sides are the same length, so this must be an equilateral triangle.
The last triangle has sides of 5.5 cm, 4.1 cm, and 8 cm. None of the sides are congruent, so this is a scalene triangle.
Equilateral triangles do not quite fit this pattern. They are always acute. This is because the three angles in an equilateral triangle always measure \begin{align*}60^{\circ}\end{align*}
There is one more thing to know about classifying triangles by their angles and sides. We can also tell whether a triangle is isosceles, scalene, or equilateral by its angles. Every angle is related to the side opposite it. Imagine a book opening. The wider you open it, the greater the distance between the two flaps. In other words, the wider an angle, the longer the side opposite it is. Therefore we can say that if a triangle has two congruent angles, it must have two congruent sides, and thus it must be isosceles. If it has three angles of different measures, then its sides are also all of different lengths, so it is scalene. Finally, an equilateral triangle, as we have seen, always has angles of \begin{align*}60^{\circ}\end{align*}
Once you know all of this information, you will find that you can classify a triangle by both its sides and its angles.
Example A
Define a scalene triangle.
Solution: A triangle where all of the side lengths are different.
Example B
Define an obtuse triangle.
Solution: A triangle where one angle is obtuse or greater than \begin{align*}180^{\circ}\end{align*}
Example C
Define an isosceles triangle
Solution: A triangle with two sides of the same length.
Now let's go back to the dilemma from the beginning of the Concept.
What type of triangle is in the pattern? Can you justify your answer?
The triangle in the pattern is an obtuse triangle. It is an obtuse triangle because it has an angle that is greater than \begin{align*}90^{\circ}\end{align*}
Vocabulary
 Acute Triangle

A triangle where all three angles are less than \begin{align*}90^{\circ}\end{align*}
90∘ .
 Right Triangle

A triangle with one \begin{align*}90^{\circ}\end{align*}
90∘ angle and two acute angles.
 Obtuse Triangle

a triangle with one angle that is greater than \begin{align*}90^{\circ}\end{align*}
90∘ .
 Equilateral Triangle
 all three side lengths and all three angles are congruent.
 Isosceles Triangle
 two side lengths are the same.
 Scalene Triangle
 all three side lengths are different
 Congruent
 means exactly the same, having the same measure.
Guided Practice
Here is one for you to try on your own.
Identify each triangle by both its sides and angles.
Solution
The first triangle is a right isosceles triangle. It has one right angle and two sides that are the same length.
The second triangle is an acute scalene triangle. All three sides are different lengths and all three angle measures are acute.
The third triangle is an obtuse scalene triangle. It has one obtuse angle and three different side lengths.
The last triangle is an obtuse isosceles. It has one obtuse angle and two side lengths that are the same.
Video Review
Practice
Directions: Classify each triangle by the describe angle measures as acute, obtuse or right.
 A triangle with three \begin{align*}60^{\circ}\end{align*}
60∘ angles.  A triangle with one \begin{align*}110^{\circ}\end{align*}
110∘ angle.  A triangle with one right angle and two acute angles.
 A triangle with one \begin{align*}130^{\circ}\end{align*}
130∘ angle.  A triangle with three acute angles.
 A triangle with a \begin{align*}90^{\circ}\end{align*}
90∘ angle.  A triangle with three angles that are less than \begin{align*}90^{\circ}\end{align*}
90∘
Directions: Identify each triangle by the side lengths described. Identify them as equilateral, isosceles or scalene.
 A triangle with side lengths of 6 in, 6 in and 4 inches.
 A triangle with side lengths of 3 ft, 4 ft, and 5 ft.
 A triangle with side lengths of 8 inches.
 A triangle with side lengths of 7 inches, 8 inches and 8 inches.
 A triangle with side lengths of 6 meters, 8 meters and 10 meters.
 A triangle with side lengths of 10 mm.
 A triangle with side lengths of 12 cm.
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Term  Definition 

Acute Triangle  An acute triangle has three angles that each measure less than 90 degrees. 
Congruent  Congruent figures are identical in size, shape and measure. 
Equilateral Triangle  An equilateral triangle is a triangle in which all three sides are the same length. 
Interior angles  Interior angles are the angles inside a figure. 
Isosceles Triangle  An isosceles triangle is a triangle in which exactly two sides are the same length. 
Obtuse Triangle  An obtuse triangle is a triangle with one angle that is greater than 90 degrees. 
Right Angle  A right angle is an angle equal to 90 degrees. 
Right Triangle  A right triangle is a triangle with one 90 degree angle. 
Scalene Triangle  A scalene triangle is a triangle in which all three sides are different lengths. 
Equilateral  A polygon is equilateral if all of its sides are the same length. 
Equiangular  A polygon is equiangular if all angles are the same measure. 
Image Attributions
Here you'll classify triangles by using angle measures and side lengths.