Published on 06/09/2008

#### Activity Overview

Use any sided polygon and Measure the exterior angles clockwise or counterclockwise. Drag any side or vertex and watch the calculation of the exterior angles stay at 360 degrees.

#### Before the Activity

Students need basic knowledge of selecting Cabri Jr. tools, Clear key, Alpha key to drag, and selecting the correct sides to calculate an exterior angle of a polygon. Stress selecting clockwise or counter-clockwise angles, and not the angles that are vertical to the interior angles.

#### During the Activity

Using Cabri Jr. create a polygon with any number of sides. F2 select the Line tool to create the polygon. Using the Segment tool makes it harder to measure the exterior angle. (NOTE--with 6 sides or more the calculator screen gets kind of full of lines and calculations.) Getting the students to measure the correct exterior angles not only reinforces exterior angles, but it also reintroduces vertical angles. While measuring the exterior angles, students must select the correct first line, vertex, and second line for each. As the measurements get displayed on the screen, encourage students to place the measurements near their angle. To calculate the sum of the exterior angles, students must organize their steps. First, choose F5 Calculate tool. Select one angle's measurement Plus the next angle's measurement. The calculation is displayed on the calculator. Encourage students to place the calculation between the two exterior angles. For the next step, select the sum of the first two angles Plus the third angle's measurement. Place this displayed sum between the 2nd and 3rd angles. As the student selects and adds the rest of the angles, always continue using the sum from the previous step. When the last sum is calculated, place the 360 in the center of the polygon. Students Clear the Calculate tool. Use Alpha hand to drag any side or vertex. The 360 degrees is maintained while different angles' measurements and sums change.

#### After the Activity

Take a 1to10. A 1to10 is a way to monitor the understanding of a class. Ask the students to 2nd quit Cabri Jr (or any APP), and have only a black curser on the calculator. Ask a question about their understanding of exterior angles. Students place a 1 to 10 on their screen. 1 equals the need for further explanation, and 10 equals total understanding of the concept. Screen Capture in TI-Navigator. (Quick Poll can also be used for 1to10)