When there is no specific straight line as the normal vector in geometry, the normal vector of plane vector can be solved by undetermined coefficient method according to the direction vector of two intersecting straight lines in the known plane (at this time, it is generally necessary to establish a spatial rectangular coordinate system). Note: 1 means that a vector is the normal vector of a plane. It only needs to indicate that the vector is perpendicular to any two non-* * straight lines in the plane, which is consistent with the judgment that the straight lines are perpendicular to the plane. 2. Using the undetermined coefficient method to find the normal vector of the plane, it is found that the three sub-coordinates in the vector have only a certain proportional relationship, not a specific value. In general, one coordinate is set as a constant, so that other coordinates can be obtained according to the proportional relationship.