Classify Triangles
Concept
A triangle is a closed shape with 3 angles, 3 sides, and 3 vertices. According to two major elements, triangles can be classified as :
i. On the basis of angles
ii. On the basis of the measurement of their sides.
Rules
How to classify a triangle:
1. Compare the lengths of the sides.
If the lengths are all the same, the triangle is equilateral.
If only the lengths of the two sides are equal, the triangle is isosceles.
If none of the sides are of equal length, the triangle is scalene.
2. Compare the angles.
If one of the angles is 90 degrees, the triangle is right.
If all the angles are less than 90 degrees, the triangle is acute.
If one of the angles is more than 90 degrees, the triangle is obtuse.
Example
Classify the given triangles as acute, obtuse, or right.
Solution
The first triangle has one obtuse angle (angle more than 90 degrees), therefore its an obtuse triangle.
The second triangle has all three acute angles (all angles less than 90 degrees), therefore its an acute triangle.
The third triangle has one right angle (angle is 90 degrees ), therefore its a right triangle.
Practice Classifying Triangles
Practice Problem 1
Classify and match.
Practice Problem 2
Two sides of a triangle are 5 centimeters long, and one side is 11 centimeters.
What kind of triangle is this?
Practice Problem 3
A triangle has three angles equal to 120°, 40°, and 20°, respectively.
What kind of triangle is this?
A triangle is a closed shape with 3 angles, 3 sides, and 3 vertices.
Equilateral Triangle – Triangle that has all three equal sides and equal angles.
Isosceles Triangle – Triangle that has two equal sides and two congruent angles.
Scalene Triangle – Triangle that has no equal sides and no congruent angles.
Right Triangle – A triangle that has one right angle.
Acute Triangle – A triangle that has all acute angles.
Obtuse Triangle – A triangle that has one obtuse angle and two acute angles.