CK-12 Foundation
Ck 12: Interactive Geometry: 6.6 Theorems Involving Similarity
Learn the many theorems about triangles that you can prove using similar triangles.
CK-12 Foundation
Ck 12: Interactive Geometry: 4.4 Theorems About Triangles
Discover some theorems about triangles.
CK-12 Foundation
Ck 12: Interactive Geometry: 4.1 Theorems and Proofs
Understand theorems and proofs as they relate to geometry.
CK-12 Foundation
Ck 12: Interactive Geometry: 1.7 the Pythagorean Theorem
Explore and discover the Pythagorean theorem.
National Council of Teachers of Mathematics
Nctm: Illuminations: Proof Without Words: Completing the Square
Use this interactive geometric proof to complete the square using geometry.
Dartmouth College
Dartmouth College: The Different Styles of Euclid and Copernicus
A history of the differences between Euclid's and Copernicus's methods of proving geometric ideas.
Maths Challenge
Maths challenge.net: Cables
This is a proof problem, considered a "three star" geometry problem requiring knowledge of proofs and properties of circles to solve this problem with cables in duct problem.
Maths Challenge
Maths challenge.net: Radical Axis
This is a "three star" geometry problem requiring knowledge of proofs and properties of circles. Students will use the given definition of radical axis and prior knowledges to prove a way to find a radical axis.
Varsity Tutors
Varsity Tutors: Hotmath: Algebra Review: Congruent Triangles
Find a quick, concise explanation of Congruent Triangles. Several examples are given and clearly explained.
ClassFlow
Class Flow: Congruency
[Free Registration/Login Required] Here are many variations of congruent triangles displayed for students to test their knowledge.
Math Is Fun
Math Is Fun: Triangles Contain 180 Degrees
Students learn about the angle-sum theorem. A definition, example, and proof are included.
Math Is Fun
Math Is Fun: Triangles
Students learn about the terminology and formulas associated with triangles. The learning resource consists of definitions, diagrams, and examples.
University of British Columbia
University of British Columbia: Proof of the Pythagorean Theorem
A geometric model that proves the Pythagorean Theorem in a step-by-step diagram that students can advance at their own pace. Requires Java.
Wolfram Research
Wolfram Math World: Postulate
This MathWorld site defines postulates, and explains how and where they are used in mathematics. The site has numerous links to related terms.
Wolfram Research
Wolfram Mathworld: Theorem
This simple MathWorld site defines theorems and briefly explains how a theorem is produced.
Cut the Knot
Cut the Knot!: Proofs in Mathematics
This site provides an explanation of mathematical proofs. It also has links to different types of proofs.
Varsity Tutors
Varsity Tutors: Hotmath: Algebra Review: Types of Triangles
Find a quick, concise explanation of the different types of triangles. Examples are given and clearly explained.
Jim Loy
Jim Loy's Mathematics Page: Congruence of Triangles
Find explanations of the ways to prove triangles are congruent. There is a link for Congruence Of Triangles, Part II to find the proofs of some ways to prove congruence of triangles from other postulates (i.e. Assume SAS prove ASA).
Michigan State University
Michigan State University: Biometrics: Hand Geometry
Students use hand geometry to prove their own identification.
Wolfram Research
Wolfram Math World: Two Column Proof
MathWorld gives a discussion of the use of two-column proofs by high school and middle school students and its relevance to mathematicians.