I'd like a user to be able to bend a line by dragging the end. The line will be an arc when bent. The length of the arc (L) needs to stay constant. The other end of the line, not being dragged, is at known point P1 and angle a1, which shouldn't change when the other end P2 is being dragged. The way I want it to work is that the angle at P2 (a2) should be such that that end of the line points towards the cursor when being dragged (Pc). The difficulty is that I can't work out P2 without knowing a2 and and I can't a2 without knowing P2. I have an expression where the only unknown is the radius r of the arc, but can't solve for it because of the complexity. I want to try to avoid numerical solutions. Has anyone done anything like this before? Is it possible to solve for r?