f(x)=1/n(E^n#i=1)(x-ai)^2=1/n(∑（x^2-2aix+ai^2))=x^2-[2x∑ai]/n+∑ai^2/n
=[x-(∑ai)/n]^2+∑ai^2/n-[(∑ai)/n]^2 所以x=(∑ai)/n时,f(x)最小,={∑ai^2-n[(∑ai)/n]^2}/n
用测量工具测量某物体的长度（各位高二童鞋进来~)
f(x)=1/n(E^n#i=1)(x-ai)^2=1/n(∑（x^2-2aix+ai^2))=x^2-[2x∑ai]/n+∑ai^2/n
=[x-(∑ai)/n]^2+∑ai^2/n-[(∑ai)/n]^2 所以x=(∑ai)/n时,f(x)最小,={∑ai^2-n[(∑ai)/n]^2}/n