Rationalise the denominators of 1/√11 and 10/√5.

To clear a root from the bottom, multiply the top and the bottom by the same number: by $\sqrt{a}$ for $\dfrac{1}{\sqrt{a}}$, and by the conjugate $a - \sqrt{b}$ for $\dfrac{1}{a + \sqrt{b}}$. The value does not change.