如何使用Open XML如何将公式插入Excel 2010工作表？

I'm using Visual Studio 2010 (VB.Net) and Open XML SDK 2.0. How do you insert a formula into an Excel 2010 worksheet? When I do this I also wish to set the CellValue property of the cell to a DBNull or EmptyString to force Excel to recalculate the cell when the user opens the workbok.I'm using Visual Studio 2010 (VB.Net) and Open