Solve a robot arm, or tell me why you cannot

A two-link arm, solved properly.

The joint angles come from the law of cosines, and the arm is then drawn from those angles, so what you see is the solution rather than an animation of one. The interesting part is the failure: push the target inside the minimum reach or beyond the maximum reach and there is no solution, so the interface says which rather than silently clamping the arm somewhere it was not asked to go.

Drag with a pointer or drive it from the keyboard. Presets include a deliberately unreachable target.