var wb = new XLWorkbook();
var ws = wb.Worksheets.Add("Formulas");

ws.Cell(1, 1).Value = "Num1";
ws.Cell(1, 2).Value = "Num2";
ws.Cell(1, 3).Value = "Total";
ws.Cell(1, 4).Value = "cell.FormulaA1";
ws.Cell(1, 5).Value = "cell.FormulaR1C1";
ws.Cell(1, 6).Value = "cell.Value";
ws.Cell(1, 7).Value = "Are Equal?";

ws.Cell(2, 1).Value = 1;
ws.Cell(2, 2).Value = 2;
var cellWithFormulaA1 = ws.Cell(2, 3);
// Use A1 notation
cellWithFormulaA1.FormulaA1 = "=A2+$B$2"; // The equal sign (=) in a formula is optional
ws.Cell(2, 4).Value = cellWithFormulaA1.FormulaA1;
ws.Cell(2, 5).Value = cellWithFormulaA1.FormulaR1C1;
ws.Cell(2, 6).Value = cellWithFormulaA1.Value;

ws.Cell(3, 1).Value = 1;
ws.Cell(3, 2).Value = 2;
var cellWithFormulaR1C1 = ws.Cell(3, 3);
// Use R1C1 notation
cellWithFormulaR1C1.FormulaR1C1 = "RC[-2]+R3C2"; // The equal sign (=) in a formula is optional
ws.Cell(3, 4).Value = cellWithFormulaR1C1.FormulaA1;
ws.Cell(3, 5).Value = cellWithFormulaR1C1.FormulaR1C1;
ws.Cell(3, 6).Value = cellWithFormulaR1C1.Value;

ws.Cell(4, 1).Value = "A";
ws.Cell(4, 2).Value = "B";
var cellWithStringFormula = ws.Cell(4, 3);

// Use R1C1 notation
cellWithStringFormula.FormulaR1C1 = "=\"Test\" & RC[-2] & \"R3C2\"";
ws.Cell(4, 4).Value = cellWithStringFormula.FormulaA1;
ws.Cell(4, 5).Value = cellWithStringFormula.FormulaR1C1;
ws.Cell(4, 6).Value = cellWithStringFormula.Value;

// Setting the formula of a range
var rngData = ws.Range(2, 1, 4, 7);
rngData.LastColumn().FormulaR1C1 = "=IF(RC[-3]=RC[-1],\"Yes\", \"No\")";

// Using an array formula:
// Just put the formula between curly braces
ws.Cell("A6").Value = "Array Formula: ";
ws.Cell("B6").FormulaA1 = "{A2+A3}";

ws.Range(1, 1, 1, 7).Style.Fill.BackgroundColor = XLColor.Cyan;
ws.Range(1, 1, 1, 7).Style.Font.Bold = true;
ws.Columns().AdjustToContents();

// You can also change the reference notation:
wb.ReferenceStyle = XLReferenceStyle.R1C1;

// And the workbook calculation mode:
wb.CalculateMode = XLCalculateMode.Auto;

wb.SaveAs("Formulas.xlsx");