Option Explicit
Sub Totaliser_A1_vers_Synthèse()
Dim o As Worksheet
Sheets("Synthèse").Range("a1").Clear
For Each o In Worksheets
If o.Name <> "Synthèse" Then o.Range("A1").Copy Destination:=Sheets("Synthèse").Range("a65536").End(xlUp).Offset(1, 0)
Next
With Sheets("Synthèse")
.Activate
.Range("a1") = Application.WorksheetFunction.Sum(Range("a:a"))
.Range(Range("a2"), Range("a2").End(xlDown)).Clear
End With
End Sub