



 
Hi Sarah, We often see the circumference $C$ of a circle of radius $r$ given as \[C = 2 \pi \; r\] and the area $A$ as \[A = \pi \; r^2 .\] Solve the first equation for $r,$ substitute into the second equation and solve for $A$ to see $A$ as a function of $C.$ Penny  


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