how to rationalize denominator

Multiply conjugates.

$\quad \dfrac1{1 + \sqrt 3 + \sqrt 5}\\ = \dfrac1{1 + \sqrt 3 + \sqrt 5} \cdot \dfrac{1 + \sqrt 3 - \sqrt 5}{1 + \sqrt 3 - \sqrt 5}\\ = \dfrac{1 + \sqrt 3 - \sqrt 5}{(1 + \sqrt 3)^2 - \sqrt5^2}\\ = \dfrac{1 + \sqrt 3 - \sqrt 5}{4 + 2\sqrt 3 - 5}\\ = \dfrac{1 + \sqrt 3 - \sqrt 5}{2\sqrt 3 - 1}\\ = \dfrac{1 + \sqrt 3 - \sqrt 5}{2\sqrt 3 - 1}\cdot \dfrac{2\sqrt 3 + 1}{2\sqrt 3 + 1}\\ = \dfrac{(1 + \sqrt 3 - \sqrt 5)(2 \sqrt 3 + 1)}{(2\sqrt 3)^2 - 1^2}\\ = \dfrac{1 + 2\sqrt 3 + 6 + \sqrt 3 - 2\sqrt{15} - \sqrt 5}{11}\\ = \dfrac{7 + 3 \sqrt 3 - \sqrt 5 - 2\sqrt {15}}{11}$