Lesson 21 — Activity 2: Sorting Regular Polygons


Getting Ready


A polygon is a closed figure consisting of three or more straight sides. In this activity you will look at different polygons and classify them by either the number of sides or the lines of symmetry.

This image shows examples of polygons.
Image courtesy of www.imagesgoogle.com 

There are many different polygons. Look at this table to see polygons from 3 sides (a triangle) to 10 sides (a decagon). For this activity, you only need to remember the names of the three- to six- sided polygons.

This table shows you many different kinds of polygons.

It is important that you remember the names of the polygons up to six sides.

  • three sides — triangle

  • four sides — quadrilateral

  • five sides — pentagon

  • six sides — hexagon


Symmetry also exists outside of quadrilaterals.

Look at these regular polygons and see how many lines of symmetry they have.


An equilateral triangle has three internal angles and three lines of symmetry.


Lines of symmetry on an equilaterial triangle

A regular pentagon has five internal angles and five lines of symmetry.


Lines of symmetry on a pentagon


A regular hexagon has six internal angles and six lines of symmetry.


Lines of symmetry on a hexagon

A regular octagon has eight internal angles and eight lines of symmetry.


Lines of symmetry on an octagon

Look at the number of sides and the number of lines of symmetry.

Do you see a pattern? In regular polygons, the number of sides equals the number of lines of symmetry.