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GeoGebraGeoGebra Classroom

Exploring the Exterior Angle Theorem

•Construct line segment DF with some point in the middle E. •Construct a triangle CDE. •Construct exterior angle DEF. •Define the measures of all of the angles, including that of the exterior angle. •Think about how the exterior angle, DEF, is related to the corresponding interior angle, DEC.
•Create another triangle JKM. •Construct exterior angles JKA, JMB, and KMC. •How are all of these exterior angles related to their interiors, for example JKM to JKA? •How are the exteriors related to the other interior angles, for example JKA to KMJ and MJK.

How is the exterior angle related to the non-corresponding interior angles? Describe the relation between each in terms of triangle ABC, with exterior angle BCD.