Answer:
The area of an octagon whose perimeter is 120 cm is 1086.4 
Step-by-step explanation:
An octagon is a polygon with eight sides. If the lengths of all the sides and the measurement of all the angles are equal, the octagon is called a regular octagon.
There is a predefined set of formulas for the calculation of perimeter, and area of a regular octagon.
The perimeter of an Octagon is given by

and the area of an Octagon is given by

We know that the perimeter is 120 cm, solving for side length (a) in the perimeter formula we get

Now, we calculate the area

Since it's a right triangle you can use the Pythagorean theorem or Trig (Soh Cah Toa).
I'll just use the Pythagorean theorem : a² + b² = c²
a = 9
b = x
c = 18
9² + x² = 18²
81 + x² = 324
x² = 324 - 81
x² = 243
x = √(243)
factor 243 to find a perfect square
x = √(3*81)
x = 9√(3)<u />
Hi there!
(3/2)2+4<span>÷2*3 = 9
Hope this helps! :)</span>