Esercizio 922 pagina 63
$\dfrac{1}{3^{\frac{1}{2}}-9^{\frac{1}{2}}}+\dfrac{1}{2\sqrt{3} }=\dfrac{1}{\sqrt{3}-\sqrt{9}}+\dfrac{1}{2\sqrt{3} }=\dfrac{1}{\sqrt{3}-3}+\dfrac{1}{2\sqrt{3} }=\dfrac{1}{\sqrt{3}-3}\cdot \dfrac{\sqrt{3}+3}{\sqrt{3}+3}+\dfrac{1}{2\sqrt{3} }\cdot\dfrac{\sqrt{3}}{\sqrt{3}}=\\
=\dfrac{\sqrt{3}+3}{3-9}+\dfrac{\sqrt{3}}{6}=\dfrac{\sqrt{3}+3}{-6}+\dfrac{\sqrt{3}}{6}=-\dfrac{\sqrt{3}}{6}-\dfrac{1}{2}+\dfrac{\sqrt{3}}{6}=-\dfrac{1}{2}$