The area A of a circle is given by
[tex]A=\pi r^2[/tex]where Pi is 3.1416 and r is the radius. In our case, we get
[tex]100\operatorname{mm}=\pi r^2[/tex]and we need to find r. In this regard, if we move Pi to the left hand side we get
[tex]\frac{100}{\pi}=r^2[/tex]then, r is given by
[tex]r=\sqrt[]{\frac{100}{\pi}}[/tex]Now, the circunference C is given by
[tex]C=2\pi\text{ r}[/tex]then, by substituting our last result into this formula, we have
[tex]C=2\pi\sqrt[]{\frac{100}{\pi}}[/tex]since square root of 100 is 10, we get
[tex]C=2\pi\frac{10}{\sqrt[]{\pi}}[/tex]we can rewrite this result as
[tex]\begin{gathered} C=\frac{2\pi\times10}{\sqrt[]{\pi}} \\ C=\frac{2\sqrt[]{\pi\text{ }}\sqrt[]{\pi}\times10}{\sqrt[]{\pi}} \end{gathered}[/tex]and we can cancel out a square root of Pi. Then, we have
[tex]C=2\sqrt[]{\pi}\times10[/tex]and the circunference is
[tex]C=20\text{ }\sqrt[]{\pi}\text{ milimeters}[/tex]