Three principles of melon and bean must be included in the common questions of mathematics in the senior high school entrance examination. One is to find the analytical formula of trajectory, the length and maximum value of trajectory path.

Guadou principle: If the distance ratio of two moving points to a fixed point is a fixed value and the included angle is a fixed angle, the trajectories of the two moving points are the same. Guadou principle is a master-slave linkage trajectory problem. The driving point is called melon and the driven point is called bean. Melons move in a straight line, and so do beans. The movement of the melon is round, so is the trajectory of the bean. The key is to make the trajectory of the driven point, and make the special point of the driven point according to the special position of the driving point, so as to connect the trajectories.