一、证明直线与平面平行一般情形下,我们在证明线、面平行时,常用其判定定理,即寻找线、线平行,但有时却难以找到平面内与已知直线平行的直线.此时,可以利用向量中的共面向量定理来证明.例1四棱锥P-ABCD中,PC⊥面ABCD,PC=2.在四边形ABCD中,∠B=∠C=90°,AB=4,CD=1,点M在PB上,且 1. Prove that the straight line is parallel to the plane In general, when we prove that the lines and faces are parallel, we often use the decision theorem to find lines and lines parallel to each other, but sometimes it is difficult to find straight lines parallel to known lines in the plane. The coplanar vector theorem in the vector can be used to prove. Example 1 In the quadrangular pyramid P-ABCD, the PC surface ABCD, PC = 2. In the quadrilateral ABCD, ∠B = ∠C = 90°, AB = 4, CD = 1, point M is on PB, and