Câu b: Tổng giai thừa

Private Function GT(n As Long) As Decimal
        Dim j As Long
        Dim kq As Decimal
        kq = 1
        For j = 1 To n
            kq = kq * j
        Next
        GT = kq
    End Function
    Private Sub CmdIn_Click(sender As Object, e As EventArgs) Handles CmdIn.Click
        Dim n As Long
        Dim kq As Decimal
        Dim i As Long
        Do While n <= 0
            n = InputBox("Nhap vao mot so nguyen duong: ", "Nhap", 0)
        Loop
        For i = 1 To n
            kq = kq + GT(i)
        Next
        MsgBox("Ket qua: " & kq, 48, "Ket qua")
    End Sub