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## 'Ab Surd Ity' printed from http://nrich.maths.org/

For the first part try squaring the whole expression.

For the second part, you could write $a = {\root 3\of
{2+\sqrt{5}}}$ and $b= {\root 3\of {2-\sqrt{5}}}$, and use the
identity $$(a + b)^3 = a^3 + 3a^2b + 3ab^2 + b^3 = a^3 + 3ab(a + b)
+ b^3.$$