条件极值问题。设将a分为a1, a2, ..., an, 构造1拉格朗日函数为L=(a1)^2+(a2)^2+...+(an)^2+λ(a1+a2+...+an-a)L'=0: 2a1+λ=0,L'=0: 2a2+λ=0.............L'=0: 2an+λ=0.L'<λ>=0: a1+a2+...+an=a由前面n个式子，得 a1=a2=...=an=-λ/2, 代入a1+a2+...+an=a， 得 λ=-2a/n则 a1=a2=...=an=a/n.
