(For full lesson w/handouts and worksheets, see attached document)
• Protractors OR Copies of Angle Measures Page (1 per student)
• Scissors (1 per student)
• Optional: Calculators & Markers or Highlighters
• Decide if you would like your students to use protractors to measure the angles of the angle reference sheet to estimate angle measures.
• Pass out triangles A-C to each student along with scissors for them to cut out the triangles (note: Triangles D-F are optional for those who finish early or if you want more variety).
• Distribute Protractors OR the Angle Measures Page and do the following steps together with Triangle A.
• Use a marker, highlighter or even a pencil to shade in about 1 inch of the interior of each angle of the triangle.
• One at a time, compare the angles to the reference angles OR have students measure with calculators.
• Ask everyone to write estimates of the angle measures in the interior of each angle.
• Have students record their measurements or estimates in the table.
• Rip off the three corners of the triangle and set them so that the angles are adjacent. (That is, so that the vertices are the same point and one side is shared.)
• Estimate the angle formed by all three angles together.
• Compare this to the total of the three earlier estimates. Make changes to the original estimates if necessary.
• Ask participants to repeat what they did to Triangle A with 2-3 other triangles and discuss with their group anything they observe.
• Randomly draw people to share observations.
• Be sure it is observed that for all of these triangles (and indeed all triangles) the three angles together make 180°.
• Show the following triangle and ask if there is enough information to figure out the measure of the last angle.
• Ask groups to discuss and then randomly draw someone to share their thoughts.