Hi. Kind of an interesting inverse function with the main function
Tan(x) - x

There are multiple values for an inverse, so we need to make some
assumptions.
If x is value Pi/2, then Tan(Pi/2) is infinity.

If given a number like 10, or 100, then the inverse is very close to Pi/2.
Therefore, if given a number like 10, the first loop of Newton's method most
likely will calculate a number on the other side of Pi/2, where the slope
reverses.
This will cause the calculation not to work.
It appears that if given a large number that is very close to a
discontinuity, it appears best to make a guess as close to Pi/2 as possible
so the next guess stays in the same quadrant.

Perhaps something like this if you think you may have numbers greater than
about 1.


Function RevInv(n)
Dim g As Double
Dim r As Double
Dim Limit As Double

Limit = WorksheetFunction.Pi / 2
If n < 1 Then n = Limit / 2 Else g = Limit - 1 / 1000000
Do While r < g
r = g
g = g - (Tan(g) - g - n) / Tan(g) ^ 2
Loop
RevInv = g
End Function

--
HTH. :)
Dana DeLouis
Windows XP, Office 2003

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