reserve a,b,c,x,y,z for Real;

theorem Th28:
  0 <= x & 0 <= y & sqrt x = sqrt y implies x = y
proof
  assume that
A1: 0 <= x and
A2: 0 <= y and
A3: sqrt x = sqrt y;
  assume x <> y;
  then x < y or y < x by XXREAL_0:1;
  hence contradiction by A1,A2,A3,Lm3;
end;
