Education Technology

Surround and Be Found! Triangular Areas, That Is!

Published on 06/09/2008

Activity Overview

In this activity, students use the Geoboard to determine the area of triangles using the surrounding rectangle technique. They also develop and use formulas to determine the area of triangles.

Before the Activity

  • Start the Geoboard App and format it
  • See the attached PDF file for detailed instructions for this activity
  • Print pages 11 - 18 from the attached PDF file for your class
  • During the Activity

    Distribute the page to the class.

    Follow the Activity procedures:

  • Construct a 2-unit by 3-unit rectangle ABCD
  • Draw a segment AC and, rename A as E and C as F
  • Observe that the rectangle is split into 2 right angle triangles and has an area of 6 square units
  • Note that the area of triangle EBF is half the area of the rectangle and is 3 square units


  • Construct a 3-unit by 4-unit right triangle and surround it by a rectangle
  • Observe that the area of the rectangle is 12 square units
  • Note that the area of the triangle is half that of the rectangle and is 6 square units


  • Draw an obtuse triangle and surround it by a rectangle
  • Find the area of the rectangle
  • Subtract the area of the two right angle triangle to find the area of the obtuse triangle within the surrounding rectangle
  • Repeat the activity to find the area of other triangles
  • After the Activity

    Review student results:

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