Education Technology

Figuring Areas

Published on 06/09/2008

Activity Overview

In this activity, students find the area of regular polygons using a summation approach and compare the summations with area calculated by Cabri Geometry Application. They also examine patterns of finding area and perimeter as the number of sides of the polygon is changed.

Before the Activity

  • See the attached PDF file for detailed instructions for this activity
  • Print pages 59 - 66 from the attached PDF file for your class
  • During the Activity

    Distribute the pages to the class.

    Follow the Activity procedures:

  • Construct a line segment
  • Construct a circle with the line segment as radius
  • Draw a regular pentagon within the circle
  • Construct a triangle using the segment as one of the sides and two additional segments
  • Find the midpoint (m) of the base of the triangle
  • Construct an altitude of the triangle (apothem of the regular polygon)
  • Measure and record length of base, height, and radius of polygon
  • Calculate and record the area of the triangle using the formula 1/2 base length (B) x height (H)
  • Observe that the polygon can be further divided into congruent triangles like the first one
  • Find the number of triangles that can be constructed in the pentagon
  • Note that the internal area of the polygon, can be calculated by the formula 1/2 (B/H) x n, where n is the number of triangles inside the polygon
  • Find the perimeter of the pentagon and then calculate the area of a pentagon using the formula: Area of polygon is = 1/2 (perimeter) x apothem
  • Use the Voyage 200 PLT to directly calculate the area of the pentagon and compare the three methods
  • Construct other polygons and find their area and perimeter and enter the values in a table
  • Observe as the number of sides of the polygon increases, the geometric shape approaches a circle
  • The perimeter of a circle is given by the formula 2 π r and area is given by the formula π r2
  • Compare the area of a circle with a polygon with many sides
  • After the Activity

    Review student results:

  • As a class, discuss questions that appeared to be more challenging
  • Re-teach concepts as necessary