Question

There is office party ,, given the radii of circle and number of guest, find the area of cake

Answer 1

This is a known problem: it is appropriately called the Circle Cutting Problem.

I suggest to you to read the linked page --- it is quite informative. However, the important part is that with n cuts, you can divide a circle into at most f(n) parts, where:

f(n)=12(n2+n+2)

In your case, f(7)=12(49+7+2)=12(58)=29.