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

radian

The angle subtended by an arc equal in length to the circle's radius.

One radian is the angle subtended at the center of a circle by an arc whose length equals the circle's radius $r$ . A full rotation is $2\pi$ radians, and radian measure is a pure, unitless number since it's a ratio of two lengths, $\theta = s/r$ .