利用 NPOI 公式指令：

HSSFWorkbook workbook = new HSSFWorkbook();
HSSFSheet sheet = (HSSFSheet)workbook.CreateSheet("工作表名稱");
HSSFRow Row = (HSSFRow)sheet.CreateRow(0);
Row.CreateCell(0).CellFormula = "SUM(A1:B1)";
Row.CreateCell(1).CellFormula = "A1-B1";
//更新有公式的欄位
sheet.ForceFormulaRecalculation = true;