Simplify the square roots in the numerator and denominator.

$$\sqrt{18} = \sqrt{9 \times 2} = 3\sqrt{2}$$ $$\sqrt{50} = \sqrt{25 \times 2} = 5\sqrt{2}$$

$$\frac{8 - 4\sqrt{18}}{\sqrt{50}} = \frac{8 - 4(3\sqrt{2})}{5\sqrt{2}}$$ $$= \frac{8 - 12\sqrt{2}}{5\sqrt{2}}$$

$$\frac{8 - 12\sqrt{2}}{5\sqrt{2}} \times \frac{\sqrt{2}}{\sqrt{2}}$$ $$= \frac{(8 - 12\sqrt{2})\sqrt{2}}{5\sqrt{2}\sqrt{2}}$$ $$= \frac{8\sqrt{2} - 12(\sqrt{2})^2}{5(2)}$$ $$= \frac{8\sqrt{2} - 12(2)}{10}$$ $$= \frac{8\sqrt{2} - 24}{10}$$

$$= \frac{8\sqrt{2}}{10} - \frac{24}{10}$$ $$= \frac{4\sqrt{2}}{5} - \frac{12}{5}$$

Comparing $-\frac{12}{5} + \frac{4}{5}\sqrt{2}$ with $a + b\sqrt{n}$:
$a = -\frac{12}{5}$
$b = \frac{4}{5}$
$n = 2$

The simplified expression is \boxed{\text{-\frac{12}{5} + \frac{4}{5}\sqrt{2}}}.
The values are \boxed{\text{a = -\frac{12}{5}$,$b = \frac{4}{5}$,$n = 2}}.