This example demonstrates how to define names for formulas. To do this, call the DefinedNameCollection.Add method with a name to be associated with a formula and the formula string passed as parameters. Use the Worksheet.DefinedNames or IWorkbook.DefinedNames property to access and modify the collection of defined names of a particular worksheet or entire workbook, depending on which scope you want to specify for a name.
C#:FormulaActions.cs 
Worksheet worksheet1 = workbook.Worksheets["Sheet1"];
Worksheet worksheet2 = workbook.Worksheets["Sheet2"];
worksheet1.DefinedNames.Add("Range_Sum", "=SUM(Sheet1!$A$1:$C$3)");
workbook.DefinedNames.Add("Range_DoubleSum", "=2*Sheet1!Range_Sum");
worksheet2.Cells["C2"].Formula = "=Sheet1!Range_Sum";
worksheet2.Cells["C3"].Formula = "=Range_DoubleSum";
worksheet2.Cells["C4"].Formula = "=Range_DoubleSum + 100";

VB:FormulaActions.vb 
Dim worksheet1 As Worksheet = workbook.Worksheets("Sheet1")
Dim worksheet2 As Worksheet = workbook.Worksheets("Sheet2")
worksheet1.DefinedNames.Add("Range_Sum", "=SUM(Sheet1!$A$1:$C$3)")
workbook.DefinedNames.Add("Range_DoubleSum", "=2*Sheet1!Range_Sum")
worksheet2.Cells("C2").Formula = "=Sheet1!Range_Sum"
worksheet2.Cells("C3").Formula = "=Range_DoubleSum"
worksheet2.Cells("C4").Formula = "=Range_DoubleSum + 100"

The image below shows how to use named formulas in worksheet cells.