Question 176636
The area of a regular decagon of side length {{{a}}} is either:


{{{A = (5/2)a^2 * cot(pi/10)}}} or


{{{A = (5/2)a^2 * sqrt(5 + 2*sqrt(5))}}}


Just plug in 4 for {{{a}}} and get to work on your calculator.


See Wikipedia (http://en.wikipedia.org/wiki/Decagon) for more info on decagons.