## Precalculus (10th Edition)

$19.90$ square units
By Heron's formula, $K=\sqrt{s(s-a)(s-b)(s-c)}$, where $s=\frac{a+b+c}{2}.$ Hence, $s=\frac{5+8+9}{2}=11$ Thus, $K=\sqrt{11(11-5)(11-8)(11-9)}\approx19.90.$