PM> Install-Package Spire.XLS

using Spire.Xls;
using System;

namespace InsertFormulasAndFunctions
{
    internal class Program
    {
        static void Main(string[] args)
        {
            //Workbookのオブジェクトを作成する
            Workbook workbook = new Workbook();

            //最初のワークシートを取得する
            Worksheet worksheet = workbook.Worksheets[0];

            //currentRowとcurrentFormulaの2つの変数を宣言する
            int currentRow = 1;
            String currentFormula = null;

            //列の幅を設定する
            worksheet.SetColumnWidth(1, 32);
            worksheet.SetColumnWidth(2, 16);

            //セルにコンテンツを書き込む
            worksheet.Range[currentRow, 1].Value = "試験データ";
            worksheet.Range[currentRow, 2].NumberValue = 1;
            worksheet.Range[currentRow, 3].NumberValue = 2;
            worksheet.Range[currentRow, 4].NumberValue = 3;
            worksheet.Range[currentRow, 5].NumberValue = 4;
            worksheet.Range[currentRow, 6].NumberValue = 5;
            currentRow += 2;
            worksheet.Range[currentRow, 1].Value = "数式または関数";
            worksheet.Range[currentRow, 2].Value = "結果";

            //セルの書式を設定する
            CellRange range = worksheet.Range[currentRow, 1, currentRow, 2];
            range.Style.Font.FontName = "Yu Gothic UI";
            range.Style.KnownColor = ExcelColors.LightGreen;
            range.Style.FillPattern = ExcelPatternType.Solid;
            range.Style.Borders[BordersLineType.EdgeBottom].LineStyle = LineStyleType.Medium;
            range.Style.Font.IsBold = true;

            //算術計算
            currentFormula = "=1/2+3*4";
            worksheet.Range[++currentRow, 1].Text = "'" + currentFormula;
            worksheet.Range[++currentRow, 2].Formula = currentFormula;

            //日付関数
            currentFormula = "=TODAY()";
            worksheet.Range[++currentRow, 1].Text = "'" + currentFormula;
            worksheet.Range[currentRow, 2].Formula = currentFormula;
            worksheet.Range[currentRow, 2].Style.NumberFormat = "YYYY/MM/DD";

            //三角関数
            currentFormula = "=SIN(PI()/6)";
            worksheet.Range[++currentRow, 1].Text = "'" + currentFormula;
            worksheet.Range[currentRow, 2].Formula = currentFormula;

            //平均関数
            currentFormula = "=AVERAGE(B1:F1)";
            worksheet.Range[++currentRow, 1].Text = "'" + currentFormula;
            worksheet.Range[currentRow, 2].Formula = currentFormula;

            //合計関数
            currentFormula = "=SUM(B1:F1)";
            worksheet.Range[++currentRow, 1].Text = "'" + currentFormula;
            worksheet.Range[currentRow, 2].Formula = currentFormula;

            //ワークブックを保存する
            workbook.SaveToFile("数式と関数の追加.xlsx", FileFormat.Version2013);
        }
    }
}

Imports Spire.Xls
Imports System

Namespace InsertFormulasAndFunctions
    Friend Class Program
        Shared Sub Main(ByVal args() As String)
            'Workbookのオブジェクトを作成する
            Dim workbook As Workbook = New Workbook()

            '最初のワークシートを取得する
            Dim worksheet As Worksheet = workbook.Worksheets(0)

            'currentRowとcurrentFormulaの2つの変数を宣言する
            Dim currentRow As Integer = 1
            Dim currentFormula As String = Nothing

            '列の幅を設定する
            worksheet.SetColumnWidth(1, 32)
            worksheet.SetColumnWidth(2, 16)

            'セルにコンテンツを書き込む
            worksheet.Range(currentRow, 1).Value = "試験データ"
            worksheet.Range(currentRow, 2).NumberValue = 1
            worksheet.Range(currentRow, 3).NumberValue = 2
            worksheet.Range(currentRow, 4).NumberValue = 3
            worksheet.Range(currentRow, 5).NumberValue = 4
            worksheet.Range(currentRow, 6).NumberValue = 5
            currentRow += 2
            worksheet.Range(currentRow, 1).Value = "数式または関数"
            worksheet.Range(currentRow, 2).Value = "結果"

            'セルの書式を設定する
            Dim range As CellRange = worksheet.Range(currentRow, 1, currentRow, 2)
            range.Style.Font.FontName = "Yu Gothic UI"
            range.Style.KnownColor = ExcelColors.LightGreen
            range.Style.FillPattern = ExcelPatternType.Solid
            range.Style.Borders(BordersLineType.EdgeBottom).LineStyle = LineStyleType.Medium
            range.Style.Font.IsBold = True

            '算術計算
            currentFormula = "=1/2+3*4""'" + currentFormulacurrentFormula

            '日付関数
            currentFormula = "=TODAY()""'" + currentFormula
            worksheet.Range(currentRow, 2).Formula = currentFormula
            worksheet.Range(currentRow, 2).Style.NumberFormat = "YYYY/MM/DD"

            '三角関数
            currentFormula = "=SIN(PI()/6)""'" + currentFormula
            worksheet.Range(currentRow, 2).Formula = currentFormula

            '平均関数
            currentFormula = "=AVERAGE(B1:F1)""'" + currentFormula
            worksheet.Range(currentRow, 2).Formula = currentFormula

            '合計関数
            currentFormula = "=SUM(B1:F1)""'" + currentFormula
            worksheet.Range(currentRow, 2).Formula = currentFormula

            'ワークブックを保存する
            workbook.SaveToFile("数式と関数の追加.xlsx", FileFormat.Version2013)
        End Sub
    End Class
End Namespace