Function Derive(ByVal Equation As String, Optional ByVal X As Variant = 0) As Double
'Calcul de la dérivé en un point : f'(x)

'Paramètres
Dim H As Double, f1 As Double, f2 As Double
'Algo
H = 0.00001
f1 = F(Equation, X - H)
f2 = F(Equation, X + H)
Derive = (f2 - f1) / (2 * H)

End Function