To set up a straight line at right angles to a given plane from a given point in it.
从给定平面内的一点作该平面的垂线。
Let the plane of reference be the given plane, and A the point in it; thus it is required to set up from the point A a straight line at right angles to the plane of reference.
设给定平面为参考平面，A为其中一点，需从A作参考平面的垂线。
Let any elevated point B be conceived, from B let BC be drawn perpendicular to the plane of reference, [XI. 11] and through the point A let AD be drawn parallel to BC.
设想平面外一点B，从B作BC垂直于参考平面（依据XI.11）。
[I. 31] Then, since AD, CB are two parallel straight lines, while one of them, BC, is at right angles to the plane of reference, therefore the remaining one, AD, is also at right angles to the plane of reference.
过A作AD平行于BC（依据I.31）。
由于AD、CB是两平行直线，且BC垂直于参考平面，故AD也垂直于参考平面（依据XI.8）。