If $\sqrt{8.1}$ is represented on the number line, then find the value of OA and OC.

It is given that $\sqrt{8.1}$ is represented on the number line.

In order to represent $\sqrt{8.1}$ on the number line, we first draw a line segment AB of length 8.1 units and extend it to C such that BC = 1. 

Let O be the midpoint of AC and with O as center and OA as radius, draw a semicircle that cuts AB at C. 

AC=AB+BC

Moreover, AB = 8.1 units and BC = 1 unit

Therefore, AC = 9.1 units

Also, AC = 2 OA 

Therefore, $\mathbf{OA}\mathbf{=}\mathbf{ }\mathbf{OC}\mathbf{ }\mathbf{=}\mathbf{ }\frac{\mathbf{AC}}{\mathbf{2}}\mathbf{ }\mathbf{=}\mathbf{ }\mathbf{ }\frac{\mathbf{9}\mathbf{.}\mathbf{1}}{\mathbf{2}}\mathbf{ }\mathbf{=}\mathbf{ }\mathbf{4}\mathbf{.}\mathbf{55}\mathbf{ }\mathbf{units}$.