面白い問題おしえて〜な 39問目

(2)
両辺を3乗すると
　(y+1) - (y-1) - 3(yy-1)^(1/3){(y+1)^(1/3) - (y-1)^(1/3)} = 1^3,
　2 - 3(yy-1)^(1/3) = 1,
　(yy-1)^(1/3) = 1/3,
　yy - 1 = 1/(3^3) = 1/27,
　y = ±√(28/27) = (2/9)√21,