In the vector part, we often come across a model called paw model. Many students are very familiar with the conclusion memory, but they don't understand the essence of paw model very well. At the same time, many students are not familiar with the application of paw model. In fact, the claw model comes from the three-point * * line theorem of plane vector. It should be the projection theorem. The so-called projection is the orthographic projection.

Among them, the vertical foot from a point to a straight line is called the orthogonal projection of this point on this straight line. The line segment between the orthographic projections of two endpoints of a line segment on a straight line is called the orthographic projection of this line segment on this straight line.