[tex]\dfrac 1{\sqrt 7- \sqrt 3}\\\\\\=\dfrac{\sqrt 7 + \sqrt 3}{\left ( \sqrt 7 -\sqrt 3\right) \left( \sqrt 7 + \sqrt 3\right)}\\\\\\=\dfrac{\sqrt 7 + \sqrt 3}{\left(\sqrt 7 \right)^2 - \left(\sqrt 3 \right)^2}\\ \\\\=\dfrac{\sqrt 7 + \sqrt 3}{7-3}\\\\\\=\dfrac 14\left(\sqrt 7 + \sqrt 3\right)[/tex]
