## GeoGebra applets, lessons, and resources.

Move points *A*, *B*, and *C* to choose the triangle size and shape you want. Move sliders and to the extreme right.

- What do you observe about angles
*1*, *2* and *3*?
- What is the sum of angles
*1*, *2*, and *3*? Justify your answer.
- Based on your observations above, what can you say about the sum of the interior angles of a triangle? Explain your answer.