reserve x,y,t for Real;

theorem Th15:
  0<x & x<1 implies (1+x)/(1-x)>0
proof
  assume that
A1: 0<x and
A2: x<1;
  x^2<x by A1,A2,SQUARE_1:13;
  then x^2<1 by A2,XXREAL_0:2;
  hence thesis by Lm4;
end;
