Function ArcCos(a As Double) As Double
    If a = 1 Then
        ArcCos = 0
    Else
        ArcCos = 2 * Atn(1) - Atn(a / Sqr(1 - a * a))
    End If
End Function
 
Function ArcSin(a As Double) As Double
    ArcSin = Atn(a / Sqr(1 - a * a))
End Function