reserve x,y,t for Real;

theorem Th13:
  x^2<1 implies (x^2+1)/(1-x^2)>=0
proof
  assume x^2<1;
  then
A1: 0<1-x^2 by XREAL_1:50;
  x^2>=0 by XREAL_1:63;
  hence thesis by A1;
end;
