[tex]\text {Circumference = } 2 \pi r[/tex]
[tex]c = 2 \pi r[/tex]
[tex]\text {r = } \dfrac{c}{2 \pi } [/tex]
[tex]\text {area = } \pi r^2[/tex]
Sub r into area:
[tex]\text {area = } \pi ( \dfrac{c}{2 \pi } )^2[/tex]
[tex]\text {area = } \dfrac{\pi c^2}{4 \pi^2 } [/tex]
[tex]\text {area = } \dfrac{\ c^2}{4 \pi } [/tex]
