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

13.1  Similar Triangles

http://www.math.com/school/subject1/lessons/S1U2L4GL.html
This site defines similar figures.

http://www.gcseguide.co.uk/similar_triangles.htm
This page gives a brief description of similar triangles.

13.2  The Pythagorean Theorem

http://www.cut-the-knot.org/pythagoras/index.shtml
This page defines the Pythagorean Theorem and has several proofs explaining it.

http://www.mathsisfun.com/pythagoras.html
This site explains the Pythagorean Theorem and has pictures showing the sides of a triangle.

13.3  Irrational Numbers and Radicals

http://www.themathpage.com/aTrig/Radicals.htm
This page contains an explanation of radicals, rational, and irrational numbers and had questions with interactive answers.

13.4  Special Right Triangles

http://www.onlinemathlearning.com/special-right-triangles.html
This site explains the special right triangles and tells the lengths of their sides.

http://www.regentsprep.org/Regents/Math/rtritrig/Ltri45.htm
This page explains the 45-45-90 special right triangle.

http://regentsprep.org/Regents/mathb/5B1/Tri30.htm
This page explains the 30-60-90 special right triangle.

13.5  Trigonometric Ratios

http://www.tpub.com/math1/20b.htm
This site defines what trigonometric ratios are, gives the six basic, and states their abbreviations.

http://mathsfirst.massey.ac.nz/Trig/TrigRatio.htm
This page stages the six most common trigonometric ratios and has an interactive triangle to practice deciphering one ratio from the other.

13.6  Solving Equations Containing Radicals

http://216.253.94.53/Algebra/radicalequ.html
This page explains how to solve equations containing radicals and has examples.

http://www.cliffsnotes.com/WileyCDA/CliffsReviewTopic/Solving-Radical-Equations.topicArticleId-38949,articleId-38937.html
This site explains how to solve equations where there is a variable under the radical.