theorem Th20:
  r1*r2=((r1+r2)/2)^2 iff r1 = r2
proof
   hereby
     assume r1*r2=((r1+r2)/2)^2;
     then sqrt( r1*r2) = (r1+r2)/2 by SQUARE_1:22;
     hence r1=r2 by Th19;
   end;
   thus thesis;
end;
