In fact, the reason is simple:
You must know two things:
1, the angle xzy: z is any point on the arc xy,
In this question, the angle xzy can be understood as the angle of the circle corresponding to the optimal arc between two points (two lighthouses) on a circle, and the size is constant (why not? Think about it:))
2. Just at any point outside the circle, the xy angle of view of the ship P and the two lighthouses will be smaller than xzy, so there will be no danger. If the xy angle of view of the ship P and the two lighthouses is larger than xzy, then the ship has already entered the circle and will be in danger.