a>b>0 <=>a-b>0.(a-b)/(a^2+b^2)=(a-b)/[(a-b)^2+2] =1/[(a-b)+2/(a-b)] <=(分子、分母同时除以(a-b)).<=1/{2√[(a-b)*2/(a-b)} (当(a-b)=2/(a-b)时，取等号）.=1/(2√2) (a-b=√2时取等号)=>最大值为√2/4.
