hey!!
here is your answer >>>
As being a amphitheatre, it has to be a 3D shape, And this could also be called a sphere, and the area to be found can be compared to the volume, and the formula for the volume a sphere is ,
(4/3)π(r)^3
radius would be 50ft , as the diameter is 100 ft.
and the area or the volume could hence be caluculated!.
Hope my answer helps!