公式：a³±b³=(a±b)(a²-+ab+b²)
=[x^(1/3)-1][x^(2/3)+x^(1/3)+1]/[x^(1/3)-1]-[2x^(1/3)+1][4x^(2/3)-2x^(1/3)+1]/[4x^(2/3)-2x^(1/3)+1]
=x^(2/3)+x^(1/3)+1-2x^(1/3)-1
=x^(2/3)-x^(1/3)