We start with a simple idea.

 

 Move B on the circle! What is the track of the midpoint M between A and B?

Select the track tool. Then click on M, because we want the track of this point. Then move B with a left mouse button drag.

The automatic track tool does the same automatically. Select it, then click on M, then on B. B is bound on the circle, so it will automatically move on it.

You can stop the animation of the track with a left mouse button click.

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