Formulas

var workbook = new Workbook();
var sheet = workbook.Sheets.AddSheet("Sheet1");

sheet["A1"].Value = "Fibonacci Numbers";
sheet["A2"].Value = 1;
sheet["A3"].Value = 1;
for (var r = 3; r < 100; r++)
    sheet[r, 0].Formula = String.Format("={0}+{1}", Cell.Format(r - 1, 0), Cell.Format(r - 2, 0));

//instead of setting formula for each cell it is easier to apply shared formula to a cell range

sheet["B2"].Value = 1;
sheet["B3"].Value = 1;
sheet["B4", "B100"].SetFormula("=B2+B3");

sheet["D1"].Value = "Roots";

var roots = sheet["D2", "F100"];
roots.GetColumns(1, 3).SetStyle(new CellStyle { Format = "0.000" });

foreach (var cdp in roots.GetColumn(0))
    cdp.Data.Value = cdp.Cell.Row + 1;

roots.GetColumn(1).SetFormula("=D2^(1/2)");
roots.GetColumn(2).SetFormula("=D2^(1/3)");
roots.GetColumn(3).SetFormula("=D2^(1/4)");
workbook.Save("Formulas");