theorem Th100:
  (not x in L) & L is being_line implies ex P st x in P & L c= P
  & P is being_plane
proof
  consider P being Element of plane_of_REAL n such that
A1: x in P & L c= P by Th99;
  assume ( not x in L)& L is being_line;
  then consider x1,x2 being Element of REAL n such that
A2: L = Line(x1,x2) and
A3: x - x1,x2 - x1 are_lindependent2 by Th55;
  take P;
  x1 in L & x2 in L by A2,EUCLID_4:9;
  then P = plane(x1,x,x2) by A1,A3,Th92;
  hence thesis by A1,A3;
end;
