Editing Nonagon (section)

== Regular nonagon ==
A ''[[regular polygon|regular]] nonagon'' is represented by [[Schläfli symbol]] {9} and has [[internal angle]]s of 140°. The area of a regular nonagon of side length ''a'' is given by
:<math>A = \frac{9}{4}a^2\cot\frac{\pi}{9}=(9/2)ar = 9r^2\tan(\pi/9) </math>
:::<math>= (9/2)R^2\sin(2\pi/9)\simeq6.18182\,a^2,</math>

where the radius ''r'' of the [[inscribed circle]] of the regular nonagon is

:<math>r=(a/2)\cot(\pi/9)</math>

and where ''R'' is the radius of its [[circumscribed circle]]:

:<math>R = \sqrt{(a/2)^2 + r^2 }=r\sec(\pi/9)=(a/2)\csc(\pi/9).</math>