Why is the sum always 180°?

What is the sum of the angles in a triangle? This applet demonstrates how you can find out without using a protractor.

  1. Move the vertices to create a triangle.
  2. Slide "shrinker" to the right.
  3. Notice that the triangle gets smaller, but the angles stay the same.
  4. Eventually, the triangle gets so small that it disappears, leaving just the three angles.
  5. What shape do the three angles make?





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