>>458
Wolfram alpha で n=4 の場合を計算させたら

x = (54 a^3 - 54 a^2 b - 54 a^2 c - 54 a^2 d + sqrt((54 a^3 - 54 a^2 b - 54 a^2 c - 54 a^2 d - 54
a b^2 + 108 a b c + 108 a b d - 54 a c^2 + 108 a c d - 54 a d^2 + 54 b^3 - 54 b^2 c - 54 b^2 d -
54 b c^2 + 108 b c d - 54 b d^2 + 54 c^3 - 54 c^2 d - 54 c d^2 + 54 d^3)^2 + 4 (24 (a b + a c +
a d + b c + b d + c d) - 9 (a + b + c + d)^2)^3) - 54 a b^2 + 108 a b c + 108 a b d - 54 a c^2 +
108 a c d - 54 a d^2 + 54 b^3 - 54 b^2 c - 54 b^2 d - 54 b c^2 + 108 b c d - 54 b d^2 + 54 c^3
- 54 c^2 d - 54 c d^2 + 54 d^3)^(1/3)/(12 2^(1/3)) - (24 (a b + a c + a d + b c + b d + c d) -
9 (a + b + c + d)^2)/(6 2^(2/3) (54 a^3 - 54 a^2 b - 54 a^2 c - 54 a^2 d + sqrt((54 a^3 - 54 a^2 b
- 54 a^2 c - 54 a^2 d - 54 a b^2 + 108 a b c + 108 a b d - 54 a c^2 + 108 a c d - 54 a d^2 + 54
b^3 - 54 b^2 c - 54 b^2 d - 54 b c^2 + 108 b c d - 54 b d^2 + 54 c^3 - 54 c^2 d - 54 c d^2 + 54
d^3)^2 + 4 (24 (a b + a c + a d + b c + b d + c d) - 9 (a + b + c + d)^2)^3) - 54 a b^2 + 108 a b c
+ 108 a b d - 54 a c^2 + 108 a c d - 54 a d^2 + 54 b^3 - 54 b^2 c - 54 b^2 d - 54 b c^2 + 108 b c d - 54 b d^2 + 54 c^3 - 54 c^2 d - 54 c d^2 + 54
d^3)^(1/3)) + 1/4 (a + b + c + d)