Analysis: connecting AP, according to the meaning of the question, |AP|=|CP|, you can get | Pb | +| PA | = | Pb |+| PC | = 4 > | AB |, so the trajectory of P is an ellipse with the focus on A and B and the long axis length of 4, and the trajectory equation of the intersection point P between the vertical line in AC and BC line can be obtained.

The fastest answer, hope to adopt, thank you.