Each central angle is 




The green line bisects the isosceles triangle,
so the angle at the top is 22.5° and the base
of the right triangle is 2m, so

tan(22.5°) = 

h*tan(22.5°) = 2

h =  = 4.828427125

So the area of the whole triangle is 

A = *b*h = *4*4.828427125 = 9.656854249

The area of the whole octagon is 8 times that or

9.656854249×8 = 77.254834

Incidentally h is called the "apothem".

Edwin