（　　）の中のΣＡｋを計算してから
２乗する
しか手はないように思えます。

I am a novice to personal computer; so I have some problem with
using special character like sigma. If (Sigma Ak)**2 means
(A1+A2+A3+...+An)**2, I think that the solution is given as shown below: (x**2
means x squared)

S Ak**2 + 2*S S (Ai*Aj)

where S stands for sigma, i.e., summation symbol.

The first S is summation for k = 1 to n,
the second S is for i = 1 to (n -1),
the third S is for j = (i + 1) to n.
What do you think of this solution? Hideo Nakayam