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众所周知,平面上一些命题、定理,在空间不一定成立,例如,过直线外一点作已知直线的垂线在平面上有且只有一条,如果在空间则有无数条,而过直线外一点作已知直线的平行线有且只有一条则又可推广到空间,象这样结
As we all know, some propositions and theorems in the plane are not necessarily true in space. For example, a vertical line that is a straight line outside a straight line has only one line in the plane. If there is an infinite number of lines in the space, and a point outside the straight line, It is known that there are only one parallel line of straight lines that can be extended to space, like this knot