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    Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click
        Dim myfile As Integer = My.Computer.FileSystem.ReadAllText("c:\vb\940303.SM")
        Dim i As Integer
        For i = 2 To myfile - 1
            If myfile Mod i = 0 Then
                TextBox3.Text = myfile & "非質數"
                Exit For
            Else
                TextBox3.Text = myfile & "是質數"
            End If
        Next
    End Sub