Triangles: Classification
By sides (equilateral, isosceles, scalene) and by angles.
Key idea
By sides (equilateral, isosceles, scalene) and by angles.
In depth
Triangles are classified by sides (equilateral, isosceles, scalene) and by angles (acute, right, obtuse). Side names tell you how many sides are equal, which also tells you which angles are equal.
Worked example
A triangle with sides 5, 5, 8 is isosceles. One with angles 90°, 45°, 45° is a right isosceles triangle.
Real-world uses
- Choosing triangular bracing in bridges and roof trusses
- Classifying shapes in art and design
Common mistake
Thinking an equilateral triangle is not isosceles. It is a special case with all sides equal.
How to learn this
- Read the key idea and say it in your own words.
- Work through the worked example, then try a similar problem.
- Use the interactive lesson’s visual and mini-quiz to check the pattern.
Frequently asked questions
What is Triangles: Classification?
By sides (equilateral, isosceles, scalene) and by angles. It is a topic at the beginner level in Geometry.
Where is Triangles: Classification used in real life?
Real-world examples: Choosing triangular bracing in bridges and roof trusses; Classifying shapes in art and design.
What should I learn before Triangles: Classification?
Make sure you are comfortable with Parallel Lines and Transversals, the previous topic in Geometry.
What is a common mistake with Triangles: Classification?
Thinking an equilateral triangle is not isosceles. It is a special case with all sides equal.
How can I practise Triangles: Classification?
Open the interactive lesson, play with the visual, then answer the three-question mini-quiz. Getting two right marks the topic complete.
What comes after Triangles: Classification?
Move on to Triangle Angle Sum next.
Related beginner topics in Geometry
- Points, Lines and Planes – The undefined building blocks of geometry.
- Line Segments and Rays – A segment has two endpoints; a ray starts at a point and goes on forever.
- Angles – Two rays sharing an endpoint; measured in degrees.
- Types of Angles – Acute <90°, right 90°, obtuse between 90° and 180°, straight 180°.
- Measuring Angles with a Protractor – Align the baseline and read the scale.
- Complementary and Supplementary – Pairs summing to 90° or 180°.