Sketchpad - Sum of the Angles of a Euclidean Triangle
Choose the Straight Edge Tool. Create
by clicking down somewhere,
releasing, moving somewhere else, clicking down again, and
repeating until you have a triangle.
Choose the Arrowhead Tool and hold down the
shift key in order to choose three vertices.
The order you choose your vertices will designate a specific angle
of your triangle. Under Measure release on Angle.
Notice that Sketchpad will measure the angle that you chose and
will label it for you.
Repeat to measure the other two angles in your triangle.
Remember to click on the white background in between measurements
in order to ensure that nothing is selected.
Under Measure release on Calculate...
Click on the first angle measurement and then on plus.
Then click on the second angle measurement and then on plus.
Finally, click on the third angle measurement and then on OK.
Drag your triangle around to see what happens to the angle measurement.