Developing the Concept
Once your students can identify different polygons, move on to identifying properties of specific polygons. This is also the time to make sure that your students know and use the correct mathematical vocabulary when describing properties of polygons.
Materials: Student worksheets 1 and 2, overhead projector, ruler for each student or for pairs of students, ruler to use on overhead
Preparation: Prepare an overhead transparency of worksheets 1 and 2. Distribute the student worksheets to each child.
- Ask: We have talked about different kinds of polygons. How did we describe a triangle?
List the properties of a triangle on the board: Three-sided polygon, contains three angles or corners.
- Say: Look at worksheet 1. All these figures are triangles, but some of them have special names. Look at figure c. Use your ruler to measure the three sides of this triangle.
Demonstrate using your own ruler on the transparency.
- Ask: Are all three sides the same length?(yes)
- Say: A triangle that has all the sides the same length is called an equilateral triangle. Look at the worksheet. Are any of the other triangles equilateral? Use your ruler to check.
Allow plenty of time for students to measure, then ask for volunteers. Figure e is also equilateral.
- Ask: Did anyone think that Figure a was equilateral?
Explain that Figure a has two sides that are the same length, but the third side is a different length.
- Say: A triangle with two equal sides is called an isosceles triangle. Are there any other isosceles triangles on the worksheet? Use your ruler to check.
- Say: Figure f is isosceles.
Make sure students are clear on the difference between isosceles and equilateral triangles.
- Say: Look at the other triangles on the worksheet. These triangles have sides that are all different lengths. These are called scalene triangles.
- You can do a similar lesson with quadrilaterals, using Worksheet 2. Have students identify rectangles and squares, and point out that these polygons have right angles and that rectangles have opposite sides that are congruent, while squares have all four sides congruent.
Wrap-Up and Assessment Hints
Students need practice identifying different polygons. Have students sort groups of polygons that are oriented differently to make sure they can identify polygons however they are turned. You can also ask students to draw different polygons using a straight edge. It is also a good idea to have children draw more than one polygon of each shape using different positions. This will allow you to get a better assessment of their true understanding of the properties of each polygon.