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°):

- The opposite angles at two intersecting straight lines are equal.
- 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.

Speed: