1.如果平面外的一条直线与这个平面的一条垂线垂直,那么这直线与这个平面平行。 2.已知a、b为两异面直线,由直线a上两点A、B分别引直线b的垂线,垂足为A_1、B_1,已知AB=2,A_1B_1=1;求异面直线a、b所成的角。 3.已知三条射线SA、SB、SC所成的∠ASC=∠BSC=30°,∠ASB=45°;求平面ASC与平面BSC所成的二面角的大小。 4.巳知A、B、 C、D四个点在平面a和平面β之外,A、B、C、D在平面a上的射影是A~1、B~1、c~1、D~1,且这四点在一直线上 A、B、C、D在平 1. If a straight line outside the plane is perpendicular to a vertical line in this plane, the line is parallel to this plane. 2. It is known that a and b are two different surface straight lines, and the vertical lines from the two points A and B on the straight line a respectively refer to the vertical line of the straight line b, the foot is A_1, B_1, and it is known that AB=2 and A_1B_1=1; The angle formed by lines a, b. 3. It is known that the three rays SA, SB and SC have ∠ASC=∠BSC=30° and ∠ASB=45°; and the dihedral angle formed by the plane ASC and the plane BSC is obtained. 4. Know that the four points A, B, C, and D are outside the plane a and the plane β, and the projections of A, B, C, and D on the plane a are A~1, B~1, c~1, D ~1, and these four points in the straight line A, B, C, D in the flat