Chapter 8 - Section 8.1 - Solving Quadratic Equations by Completing the Square - Exercise Set - Page 484: 90

$5-10\sqrt{3}$

Factoring the $GCF=2$ and then cancelling the common factor between the numerator and the denominator, the given expression, $ \dfrac{10-20\sqrt{3}}{2} ,$ simplifies to \begin{array}{l}\require{cancel} \dfrac{2(5-10\sqrt{3})}{2} \\\\= \dfrac{\cancel{2}(5-10\sqrt{3})}{\cancel{2}} \\\\= 5-10\sqrt{3} .\end{array}