>>41
 (1+x)^(1/3) = 1 + (x/3) - (x/3)^2 + (5/3)(x/3)^3 - (10/3)(x/3)^4 + (22/3)(x/3)^5 - …
    ≒ 1 + (x/3) - (x/3)^2 + (x/3)^3 - …
    = 1 + x/(3+x),
これに x=1/27 を代入