What is the sum of the measures of the angles in a triangle?

Move the slider (α) to adjust the measures of the angles. Answer the questions below.

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1. Position the slider on 90°. What kind of triangle is this? Record the measures of the three angles and calculate the sum.
2. Now move the slider to 40°. How can we classify this triangle? Record the measures and calculate the sum. What is it?
3. Finally, move the slider to 130°. What do we call this kind of triangle? Record the angle measures and calculate the sum.
4. You have now calculated the sum of the angles in three different kinds of triangle. What do you suppose will be the sum of the angles in any triangle you create?
5. Move the slider to two more values and check your conjecture? Were you correct?
6. If we had a new student come to class tomorrow, how would you explain what you learned today?

Brian Halopoff, 8th grade, Sequoia Middle School, Created with GeoGebra