Circle (circumference and area)

The circumference of a circle is the distance around the circle. It can be found using two formulas.

 $c=\pi d$  

 $c=2\pi r$  

 $\pi\ can\ be\ approximated\ to\ 3.14$  

The area of a circle measures the covering of a two-dimensional object.

Use the formula:

 $A\ =\ \pi r^2$  

 $r^2\ =\ r\times r$  

 $So\ that\ means\ A=\ \pi\times r\times r$  

Sometimes the area of the circle may be known, but your radius or diameter is not known.

Using the formula for Area is key to solving for your missing piece of information.

Known:  $A\ =\ \pi r^2$  

 $A=78.5in^2$  

 $\pi\ =\ 3.14$  

 $78.5\ =\ 3.14r^2$  

 $\frac{78.5}{3.14}\ =\ \frac{3.14}{3.14}r^2$  

 $25\ =\ r^2$  

Square Root to find the answer:  $r\ =\ 5$