reserve a,b,c for positive Real,
  m,x,y,z for Real,
  n for Nat,
  s,s1,s2,s3,s4,s5 for Real_Sequence;

theorem Th6:
  x^2+y^2>=2*x*y
proof
  (x-y)^2>=0 by XREAL_1:63;
  then x^2-2*x*y+y^2+2*x*y>=0+2*x*y by XREAL_1:6;
  hence thesis;
end;
