Option Explicit
Sub Test()
Dim Wsh As Worksheet
On Error Resume Next
Set Wsh = Worksheets("Résultats")
If Err Then Err.Clear: Set Wsh = Worksheets("RESULTATS")
If Err Then MsgBox "L'onglet ""RESULTAT"" n'existe pas dans le classeur actif.", _
vbCritical, "Test": Exit Sub
If Wsh.[C1].Value <> "Coût" Then
Wsh.Columns("C").Insert
With Wsh.[C1]: .Value = "Coût": .Interior.Color = RGB(0, 128, 0): End With
End If
With Wsh.[C2].Resize(Wsh.[A1000000].End(xlUp).Row - 1)
.FormulaR1C1 = "=RC[-1]/RC[1]"
' .Value = .Value
End With
End Sub