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Some links are repeated for use with more than one lesson.

This site applies to the entire chapter.

http://www.freemathhelp.com/Lessons/Geometry_Similar_Triangles_BB.htm
This site contains a video tutorial with notes on similar triangles.

4.1  Ratio and Proportion

http://www.mathleague.com/help/ratio/ratio.htm
This site defines ratio, proportion, and rate, and has examples of each.

http://www.math.com/school/subject1/lessons/S1U2L2GL.html
This page takes an in-depth look at proportion.

4.2  Similarity

http://www.cut-the-knot.org/ctk/Similarity.shtml
This site has an article called “A Case of Similarity” that proves the similarity of triangle theorems.

4.3  Proportions in Similar Triangles

This page discusses similar triangles and their proportions.

4.4  Indirect Measurement

http://www.highpointsmath.com/sitemap/IndirectMeasurement.html
This page gives a brief summary of indirect measurement and an example.

4.5  Proportions in Right Triangles