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

variable

A symbol standing in for a number that is unknown or can vary.

A variable is a symbol, typically a lowercase letter, representing a number that is unknown, unspecified, or free to vary.