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National Council of Teachers of Mathematics
The Math Forum: Ask Dr. Math: Determining Triangle Similarity
Dr. Math responds to a question about how to determine if two triangles are similar. The Dr. goes into detail about the process and gives several examples that are worked out for you.
CK-12 Foundation
Ck 12: Geometry: Polygon Similarity Study Guide
[Free Registration/Login may be required to access all resource tools.] This study guide looks at similar polygons, scale factor (ratio), and triangle similarity.