Describing Triangles Using Sides

Equilateral triangles

These are the most ‘perfect’ looking triangles – an equilateral triangle has three sides which are all the same length.  This means that the three angles inside the triangle are the same.  Equilateral triangles can also be called equiangular triangles.  ‘Equi’ sort of means ‘equal’, and ‘angular’ obviously refers to angles, so the word means that all the angles are equal.

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In an equilateral triangle, we know that all the interior angles are the same.  This means that we can work out what each angle is by:


If you look at the diagram you should see 3 small straight lines drawn through the middles of each side.  These short straight lines indicate that the side they’re drawn through is the same length as any other sides with a short line drawn through it.

Isosceles triangles

OK, first of all, the most important thing to do with isosceles triangles is to learn how to spell that word – isosceles.  I find it easiest to remember as:

                                                          isos + celes

Now, once you’ve learnt how to spell this word, you need to know what an isosceles triangle is.  Well, it’s quite simple.  Isosceles triangles have exactly two sides the same length.  So an equilateral triangle is not an isosceles triangle for instance, because it has three equal sides, not two.

Because two of the sides are equal in length, this means that all isosceles triangles will also have two equal angles.  Looking at the diagram, you should see two angles marked out, with short straight lines drawn through them.  Like for sides, these short straight lines are used to show that angles are the same size.  The two equal angles in an isosceles triangle are always in the corners of the triangle where the two equal sides do not meet.

Scalene triangles

Scalene triangles are ones where none of the sides are the same length.  This also means that none of the angles are the same. I like to think of them as ‘messy’ triangles because nothing matches. Here’s one example of a scalene triangle: