theorem Th10:
  2/((1/a)+(1/b))<=(a+b)/2
proof
A1: (a+b)/2>=sqrt(a*b) by SERIES_3:2;
  2/((1/a)+(1/b))<=sqrt(a*b) by Th7;
  hence thesis by A1,XXREAL_0:2;
end;
