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CK-12 Foundation
Ck 12: Interactive Geometry: 4.2 Theorems About Lines and Angles
Consider two parallel lines that are intersected by a transversal, and when it crosses the two others, many different angle pairs are formed. See the special properties of the angle pairs in this interactive.
CK-12 Foundation
Ck 12: Interactive Geometry: 3.2 Asa and Aas Triangle Congruence
See how triangles are congruent where all corresponding angle pairs and sides are the same.