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 letter in a formula that stands for a number to be filled in later; the blank on the recipe card.

Examples

In $y = 0.5\,x_1 + 0.2\,x_2 + 1$ , the variables $x_1$ and $x_2$ are the two blanks; plugging in $2$ and $3$ gives $2.6$ .