Illustrating Three Basic Angle Facts

Fact: Vertical angles, which are angles opposite each other at an intersection of lines, are congruent.

Fact: Two angles that add up to a straight line will add up to 180°.

Fact: The sum of the interior angles of a triangle add up to 180°.

Go ahead and move any of the points around, and see for yourself.

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Darron Steele, Created with GeoGebra