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