Let t = (a-a1) 2+(a-a2) 2+...+(a-an) 2,

When finding the minimum value of t, a is enough. because

t=n(a^2)-2(A 1+A2+...+An)a+[(A 1)^2+(A2)^2+...+(An)^2]，

Obviously, t is a quadratic function about a.gudang.

a=-[-2(A 1+A2+...+An)]/2n=(A 1+A2+...+An)]/n

T takes the minimum value. Therefore, what is needed is:

a=(A 1+A2+...+An)]/n。