由柯西不等式
(a1+a2+a2+a3+a3+a4+.+an+a1)*[a1^2/(a1+a2)+a2^2/(a2+a3)+……+an^2/(an+a1)]>=[√(a1^2)+√(a2^2)+...+√(an^2)]^2
即
2*[a1^2/(a1+a2)+a2^2/(a2+a3)+……+an^2/(an+a1)]>=(a1+a2+……+an)^2=1
a1^2/(a1+a2)+a2^2/(a2+a3)+……+an^2/(an+a1)>=1/2
取等号时a1=a2=..=an=1/n
由柯西不等式
(a1+a2+a2+a3+a3+a4+.+an+a1)*[a1^2/(a1+a2)+a2^2/(a2+a3)+……+an^2/(an+a1)]>=[√(a1^2)+√(a2^2)+...+√(an^2)]^2
即
2*[a1^2/(a1+a2)+a2^2/(a2+a3)+……+an^2/(an+a1)]>=(a1+a2+……+an)^2=1
a1^2/(a1+a2)+a2^2/(a2+a3)+……+an^2/(an+a1)>=1/2
取等号时a1=a2=..=an=1/n