Sum of Angles of Triangle

When you press the start button the base line forming the triangle begins to move upwards towards the top-corner.

Starting from the following two assumptions it is easy to show that the sum of angles of a triangle gives the straight angle (180°):

  1. The opposite angles at two intersecting straight lines are equal.
  2. Translation does not change angles between straight lines.

Maybe, the following animation gets you started.

You can change the position of the triangle top corner by a mouse click within the box above the base line of the triangle.


α γ γ β