Since the area of a circle is [tex]S=\pi r^2[/tex] [tex]\Rightarrow[/tex] [tex]80=\pi r^2[/tex].
Now we are going to divide each side by [tex]\pi[/tex].
[tex]\frac{80}{\pi}=r^2 \\r=\pm\sqrt{\frac{80}{\pi}} \\r=\pm\frac{\sqrt{80}}{\sqrt{\pi}}\approx\pm5.05 \ cm[/tex]
Since the radius could not be negative [tex]\Rightarrow[/tex] your answer is positive [tex]5.05 \ cm[/tex].
