Having trouble entering formula

I am trying to build up a spreadsheet (excel 2007) for calculating the volume of liquid in cylindrical tanks. The tanks are horizontal, and have eliptical ends. The variables are as follows:
Cell F22 : Length of cylindrical section (L)
Cell F23 : Tank radius (R)
Cell F24 : Minor radius of eliptical end caps (r)
Cell F25 : Height of tank contents (H)
Cell F26 : Eccentricity of end caps (this is the ratio r/R)

The volume formula I am using is :
v = 2Lcot^-1(sqrt((2R/y)-1)R^2 + (1/3)e*pi(3R-y)y^2+LSQRT((2R-y)y)(y-R)

I am having trouble entering the inverse cotangent cot^-1 in excel. Can anyone advise on how I can do this.
The formula as it would be written is attached.

TIA

Correct me if I am wrong. If: cos/sin=cot=1/tan
Could cot^-1 not be replaced by: (1/TAN(1))^-1
//Ola

Thanks people.