In a user defined function, how do you reference named cells in Excel2007?
Mathematically I need to evaluate:
fa(x) = 1-tanh( (x-x0)/x1 )
Mathematically, x, x0, and x1 are numbers.

In Excel the calling argument x is (the contents of) a cell, like E11, which
is a number.
x0 and x1 are named cells, each containing a number. i.e., each is a
one-cell range

I've tried:

Function fa(x)
fa = 1-Tanh( (Range(x) - Range(x0)) / Range(x1) )
End Function

which doesn't work. The error window highlights Range and says "Compile
error: Sub or Function not defined"

and

Function fa(x)
fa = 1-Tanh( (Cell("contents",x) - Cell("contents",x0) / Cell("contents",x1) )
End Function

also doesn't work. Same error with Cell highlighted.
Can someone set me straight?
I am not well versed in Excel2007.