Portal:Chapter 2: An ancient theorem and a modern question

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Interactive
Pythagorean Theorem Proof by Community Contributor @TimAlex

Euclidia

Euclid’s Elements

Videos
Playing Sports in Hyperbolic Space

Ditching the Fifth Axiom

The Essence of Linear Algebra

Books
The Four Pillars of Geometry by John Stillwell

Linear Algebra Done Right by Sheldon Axler