9. Calculate $$\frac{1}{\sqrt{5}-2} - \frac{1}{\sqrt{5}+2}$$.

Let's solve this step-by-step:

1. We need to subtract two fractions with radicals in the denominator.
2. To subtract them, we need a common denominator. The common denominator is the product of the two denominators: $$(\sqrt{5}-2)(\sqrt{5}+2)$$.
3. This is a difference of squares pattern: $$(a-b)(a+b) = a^2 - b^2$$.
4. So, $$(\sqrt{5}-2)(\sqrt{5}+2) = (\sqrt{5})^2 - 2^2 = 5 - 4 = 1$$.
5. Now, rewrite the fractions with the common denominator:
6. The first term: $$\frac{1}{\sqrt{5}-2} \times \frac{\sqrt{5}+2}{\sqrt{5}+2} = \frac{\sqrt{5}+2}{1}$$.
7. The second term: $$\frac{1}{\sqrt{5}+2} \times \frac{\sqrt{5}-2}{\sqrt{5}-2} = \frac{\sqrt{5}-2}{1}$$.
8. Now subtract the second rewritten fraction from the first: $$\frac{\sqrt{5}+2}{1} - \frac{\sqrt{5}-2}{1}$$.
9. This is $$(\sqrt{5}+2) - (\sqrt{5}-2)$$.
10. Distribute the negative sign: $$\sqrt{5}+2 - \sqrt{5} + 2$$.
11. The $$\sqrt{5}$$ and $$-\sqrt{5}$$ cancel each other out.
12. We are left with $$2 + 2 = 4$$.

Ответ: 4