Physics lab; deriving for μs and μk

[Tuccimane]

1. Physics Lab :
Homework Statement

design, run and report an experiment to calculate A) μs B) μk between a flat bottomed object and a flat surface ( include a section called "theory" in which you formally derive an expression for μs and μk )

im basically just having troubles deriving , i measured the angle in which my object has started to slide and its about 10 degrees

2. relevant equations

The Attempt at a Solution

μs :

Fun x = Sum x
0 N = Fg x + Ffs
0 N = mgcos(theta) -μsFN
0 N = mgcos(theta) - μs(mgsin(theta))
mgcos(theta)= μs(mgsin(theta))
mgcos(theta) / mgsin(theta) = μs
μs= cot(theta)

im pretty sure I am supposed to get μs= tan theta, any clue what i did wrong S:

 

[Basic_Physics]

Fgx is the sin theta component and FN the cosine component of Fg if theta is the angle that the flat surface makes with the floor.