=[(3x^2 + 1/2sqrt5) * (x^3 - sqrt5) - (x^3 - sqrt5) * (3x^2 + 1/2sqrt5)] / (x^3 - sqrt5) = ...

тут со знаками немного напутала
=[(3x^2 + 1/2sqrt5) * (x^3 - sqrt5) - (x^3 + sqrt5) * (3x^2 - 1/2sqrt5)] / (x^3 - sqrt5) = ...