Triangle Classification
Triangles are named by their sides and by their angles.
About this topic
Triangles can be classified by their sides: equilateral (all three equal), isosceles (two equal) and scalene (none equal). They can also be classified by angles: acute (all under 90°), right (one 90° angle) and obtuse (one over 90°).
In an isosceles triangle the angles opposite the equal sides are equal too. Classification helps decide which theorems and formulas apply, such as the Pythagorean theorem for right triangles, and organises thinking about shape and symmetry.
Worked example: a triangle has sides 5, 5 and 8. Two sides are equal, so it is isosceles; checking angles shows the largest is opposite the 8 and is obtuse or acute depending on whether 5² + 5² compares with 8². Since 50 is less than 64, the triangle is obtuse. A triangle with sides 6, 8 and 10 is scalene and, because 36 + 64 = 100, right.
Engineers choose triangle types for trusses and bridges, because a triangle is rigid. Students often think a triangle can be classified only one way, but every triangle has both a side name and an angle name, such as an acute isosceles triangle. An equilateral triangle is a special case of isosceles, and it is always acute with all angles 60°. Also notice that the longest side is always opposite the largest angle, which helps check your classifications and diagrams.
Key ideas
- Equilateral, isosceles, scalene
- Acute, right, obtuse
- An equilateral triangle has three 60° angles
Quick check
Use cases: where Triangle Classification helps
- Homework: review the key idea behind Triangle Classification before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Triangle Classification without notes.
- Visual learning: play with the interactive lab to see how Triangle Classification changes when you adjust its inputs.
- Teaching a friend: explain Triangle Classification in your own words, then compare your version with the key ideas.
Number and algebra skills are used in science, engineering, budgeting and computing.
Frequently asked questions
What is Triangle Classification?
Triangles are named by their sides and by their angles.
What will I learn in Triangle Classification?
Equilateral, isosceles, scalene. Acute, right, obtuse. An equilateral triangle has three 60° angles.
Is Triangle Classification part of Mathematics?
Yes. It is one of 200 Mathematics topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Triangle Classification lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Triangle Classification?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.