G.CO.9 Vertical Angles: Quick Exploration

Use the protractor to measure angles 1 and 2 below.
What did you get for the measure of [math]\angle1[/math]? How about [math]\angle2[/math]?
Use the slider to change the sizes of angles 1 and 2. Use the protractor to measure them both again.
[math]\angle1[/math] and [math]\angle2[/math] are said to be [i]vertical [/i]angles. From your observations, what seems to be true?
Describe how you can tell whether two angles are vertical? What is true about the angle measures? Use transformations to explain your reasoning.
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– Cindy Almazan, Tim Brzezinski, GeoGebra Classroom Activities, Anthony OR 柯志明

Information: G.CO.9 Vertical Angles: Quick Exploration