# Thread: Help on a simple circle formula

1. ## Help on a simple circle formula

Hey guys, I'm stuck on how to find . How did they get it/what is it?

Question:

2. This is how I would work out the volume

A = surface area
t = thickness (0.01m)
a = angle between vertical and radius line
R = Radius of circle ( 60m)
r = horizontal radius = Rsina
s = distance along surface in meriden direction

circufrence of a ring circle = 2*pie*r = 2*pie*Rsina

Surface area is the just the sum of all the horintal circufrences ie

$\int{2*pie*Rsina ds}$ (with limits s and 0)

However easier to transform into angles to work out

ds = Rda (using angle*Radius = chord length)
0 = 0
s = pie/6

$\int{2*pie*Rsina Rda}$ (with limits pie/6 and 0)

Area = 2*pie*R^2* ( 1- sqrt(3)/2)

Mass = density*A*t

Work it out and see if its right?