Expii Library is a free collection of math and science lessons, written by students, teachers and enthusiasts. More than 1,600 people have written and improved its 8,500 explanations and 24,000 practice problems, across 7,000+ topics from arithmetic to calculus, chemistry and physics.
Many explanations of each idea
Students understand the same idea in different ways, and a textbook explains it one way. On Expii, each topic is one idea, like the slope of parallel lines or the Pythagorean theorem, and students, teachers and enthusiasts explain it in their own words, with diagrams, videos and questions to try along the way. Many topics have several explanations by different people, so you can pick the one that makes sense to you, and readers vote for the ones that help them most.
Every explanation names the people who wrote it, and the Contributors page shows what each person has written.
Free to read and reuse
Every lesson is free to read, with no account needed. The lessons are shared under the Creative Commons Attribution-ShareAlike 4.0 license, so teachers and students can copy and adapt them, as long as they credit the authors and share their versions the same way. The Terms of Service have the details.
Part of Expii
Expii Library is one of Expii’s projects. Expii also runs LIVE, where young math stars teach live classes while professional actors and comedians coach them, the LIVE Math Scholarship, and the Future After AI Tour. The Expii home page tells the whole story.
Questions about using the site? The FAQ has answers.