Other
Platonic Realms: Conic Sections
A proof to show that the square root of two is an irrational number, followed by a shorter and even more beautiful proof.
TED Talks
Ted: Ted Ed: How to Prove a Mathematical Theory
Explore mathematical proofs and how they are used in our everyday life with this animated video. [4:39]
Khan Academy
Khan Academy: Inscribed Angle Theorem Proof
Proving that an inscribed angle is half of a central angle that subtends the same arc.
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.
Curated OER
Mathematical the Four Color Theorem and Computer Proof
This site describes the role of computer involvement in the four color map theorem.
Curated OER
Mathematical the Four Color Theorem and Computer Proof
This site describes the role of computer involvement in the four color map theorem.
Math Is Fun
Math Is Fun: Triangles Contain 180 Degrees
Students learn about the angle-sum theorem. A definition, example, and proof are included.
Cuemath
Cuemath: 30 60 90 Triangle
In this lesson, learn all about the 30-60-90 triangle. Learn about its sides, its area, and the rules that apply to these triangles. Includes simulations and practice questions.
CK-12 Foundation
Ck 12: Geometry: Parallelograms Study Guide
This study guide on parallelograms covers types of quadrilaterals, properties of parallelograms and theorems that describe them, how to identify parallelograms, and special types of parallelograms. It is available for download with free...
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.
Cut the Knot
Cut the Knot!: The Law of Cosines (Cosine Rule)
This site from Cut The Knot provides an overview of the law of cosines, complete with an explanation of the law and a proof of the law.
Other
Antonio Gutierrez: Geometry From the Land of the Incas
Discovering the properties of constructions, seeing proofs in action, and viewing problems come to life here with this wonderful use of technology in the classroom. Single steps are explained and illustrated one at a time so that the...
Alabama Learning Exchange
Alex: Growing in a Triangle
Middle school students explore the Pythagorean Theorem by measuring and calculating diagonal lengths using the Pythagorean Theorem. Students view an animated proof of the Pythagorean Theorem. They access an Internet site to view and...
Math Open Reference
Math Open Reference: Biography of Pythagoras
Biography of Greek mathematician, Pythagoras, who lived from 569 BC to approximately 475 BC. His theory, known as the Pythagorean Theorem continues to be of interest to mathematicians and has more than 400 different proofs.
Cut the Knot
Cut the Knot: Existence of the Orthocenter
This site defines the altitude of a triangle and gives the standard notation for altitudes of a triangle. The site then goes to list several proofs involving altitudes and orthocenters.
National Council of Teachers of Mathematics
The Math Forum: Ask Dr. Math: Proof the Inscribed Angle Theorem
Dr. Math gives a student a hint on how to prove that an inscribed angle is equal to 1/2 the central angle. You should know what the central angle and inscribed angle are to utilize this site.
Mathematics Archives
Math Archives: Visual Calculus: Comparison Test
Visual Calculus briefly states the comparison test theorem with the proof and exercise solutions as links.
Curated OER
Math Open Reference: Biography of Pythagoras
Biography of Greek mathematician, Pythagoras, who lived from 569 BC to approximately 475 BC. His theory, known as the Pythagorean Theorem continues to be of interest to mathematicians and has more than 400 different proofs.