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The formula

$\displaystyle{A=\frac{1}{2}\,r\,n\,s}$

gives the area of a regular polygon with side length $s$ , number of sides $n$ and, apothem $r$ . (The apothem is the distance from the center of the polygon to one of its sides.)

What is the area of a regular pentagon with $s= {21\ {\rm in}}$ and $r={20\ {\rm in}}$ ?

The area is . (Use in for inches and in^2 for square inches.)