Free public beta until December 4, 2026: every course is open, no card needed. After that, the first chapter of each course stays free and full access is a one-time $39 ($19 for up to 100 founding members chosen for helpful feedback). Your progress is kept. How the beta works

probability axioms

The three rules -- nonnegativity, normalization, additivity -- every probability measure must satisfy.

Nonnegativity: $P(E) \geq 0$ . Normalization: $P(S)=1$ . Additivity: mutually exclusive events' probabilities add.