※ 引述《FAlin (FA（バルシェ应援）)》之铭言：
: 4. Find all functions f: Z→Z, such that for all a+b+c = 0 holds :
: f(a)^2 + f(b)^2 + f(c)^2 = 2f(a)f(b) + 2f(b)f(c) + 2f(c)f(a)
: 5. Let △ABC be a triangle with ∠C = 90度 and that D be the foot of the
: altitude from C. Let X be a point in the interior of the segment CD.
: Let K be the point on the segment AX such that BK = BC. Similarly,
: let L be the point on the segment BX such that AL = AC.
: Let M be the point of intersection of AL and BK.
: Show that MK = ML.
: 6. Determine all positive integers n for which there exist non-negative
: integers a_1, a_2, ...,a_n such that:
: 1 1 1 1 2 n
: