1. The principle that the first two can and the last one can't be is the basic theorem of plane vectors, that is, if e 1 and e2 are two non-* line vectors in the same plane, then any vector A in the plane has only a pair of ordered real numbers (x, y), so A = Xe1+Ye2.

2.PO=xPA+yPB+zPC is right, AP=xAO+yAB+zAC is wrong, and OP=xPA+yOB+zPC is wrong.

The principle is the same as the first question, except that the plane is analogized to space.

If there must be a starting point, the starting point must be p.