theorem Th28:
  X is being_plane & M is being_line & a in X & M c= X implies a*M c= X
proof
  assume that
A1: X is being_plane and
A2: M is being_line and
A3: a in X & M c= X;
  set N = a*M;
  a in N & M // N by A2,Def3;
  hence thesis by A1,A3,Th23;
end;
