The discussion about point B 1 is as follows.

Can explain, for reference. Let q and B 1 coordinate and BQ=B 1Q, and the equation is a binary quadratic equation and a conic. The value range of this problem is part of this curve. This junior high school is not easy to understand. It can be explained schematically by graphic method, as shown in the figure, when the Q point is fixed (assuming that the P point is not restricted), the turning point of the B point is a circle with Q as the center and QB as the radius and its interior; When point Q moves to point A, the turning range of point B gradually expands. According to the meaning of the question, the turning point of point B should be on the straight line CR. As can be seen from the figure, when the circle Q moves to point A, it passes through point B, and the radius increases continuously. The part on the straight line CR is b1r >; MR, so when point Q reaches point A, B 1 is the farthest point. I am an analogy.