Как подключить dll, в которой реализуется вычисления тригонометрических функций - C#

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
 
namespace MathFunctionF
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
 
        private void button3_Click(object sender, EventArgs e)
        {
 
        }
 
        private void button1_Click(object sender, EventArgs e)
        {
            progressBar1.Value = 0;
            timer1.Enabled = true;
            double a, b, res;
 
            a = Convert.ToDouble(textBox1.Text);
            b = Convert.ToDouble(textBox2.Text);
 
            if (comboBox1.SelectedIndex == 0)
            {
                res = a + b;
                textBox3.Text = Convert.ToString(res);
            }
            if (comboBox1.SelectedIndex == 1)
            {
                res = a - b;
                textBox3.Text = Convert.ToString(res);
            }
            if (comboBox1.SelectedIndex == 2)
            {
                res = a * b;
                textBox3.Text = Convert.ToString(res);
            }
            if (comboBox1.SelectedIndex == 3)
            {
                if (b == 0)
                    MessageBox.Show("Ошибка деления на ноль!");
                else
                {
                    res = a / b;
                    textBox3.Text = Convert.ToString(res);
                }
            }
            if (comboBox1.SelectedIndex == 4)
            {
                if (b == 0)
                    MessageBox.Show("Ошибка деления на ноль!");
                else
                {
                    res = a % b;
                    textBox3.Text = Convert.ToString(res);
                }
            }
        }
 
        private void button2_Click(object sender, EventArgs e)
        {
            progressBar2.Value = 0;
            timer2.Enabled = true;
 
            double a, b, res;
 
            a = Convert.ToDouble(textBox6.Text);
            a = a *  Math.PI / 180;
            if (comboBox2.SelectedIndex == 0)
            {
                res = Math.Abs(a);
                textBox4.Text = Convert.ToString(res);
            }
            if (comboBox2.SelectedIndex == 1)
            {
                res = Math.Acos(a);
                res = Math.Round(res, 5);
                textBox4.Text = Convert.ToString(res);
            }
            if (comboBox2.SelectedIndex == 2)
            {
                res = Math.Asin(a);
                res = Math.Round(res, 5);
                textBox4.Text = Convert.ToString(res);
            }
            if (comboBox2.SelectedIndex == 3)
            {
                res = Math.Atan(a);
                res = Math.Round(res, 5);
                textBox4.Text = Convert.ToString(res);
            }
            if (comboBox2.SelectedIndex == 4)
            {
                b = Convert.ToDouble(textBox5.Text);
                res = Math.Atan2(a, b);
                textBox4.Text = Convert.ToString(res);
            }
            if (comboBox2.SelectedIndex == 5)
            {
                res = Math.Cos(a);
                res = Math.Round(res, 5);
                textBox4.Text = Convert.ToString(res);
            }
            if (comboBox2.SelectedIndex == 6)
            {
                res = Math.Sin(a);
                res = Math.Round(res, 5);
                textBox4.Text = Convert.ToString(res);
            }
            if (comboBox2.SelectedIndex == 7)
            {
                res = Math.Tan(a);
                res = Math.Round(res, 5);
                textBox4.Text = Convert.ToString(res);
            }
            if (comboBox2.SelectedIndex == 8)
            {
                res = Math.E;
                textBox4.Text = Convert.ToString(res);
            }
            if (comboBox2.SelectedIndex == 9)
            {
                res = Math.PI;
                textBox4.Text = Convert.ToString(res);
            }
            if (comboBox2.SelectedIndex == 10)
            {
                res = Math.Log10(a);
                textBox4.Text = Convert.ToString(res);
            }
            if (comboBox2.SelectedIndex == 11)
            {
                res = Math.Round(a);
                textBox4.Text = Convert.ToString(res);
            }
            if (comboBox2.SelectedIndex == 12)
            {
                res = Math.Sqrt(a);
                textBox4.Text = Convert.ToString(res);
            }
            if (comboBox2.SelectedIndex == 13)
            {
                res = Math.Truncate(a);
                textBox4.Text = Convert.ToString(res);
            }
        }
 
        private void comboBox2_SelectedIndexChanged(object sender, EventArgs e)
        {
            if (comboBox2.SelectedIndex == 4)
                textBox5.Enabled = true;
            else
                textBox5.Enabled = false;
        }
 
        private void label1_Click(object sender, EventArgs e)
        {

        }
 
        private void Form1_Load(object sender, EventArgs e)
        {
 
        }
 
        private void timer1_Tick(object sender, EventArgs e)
        {
            progressBar1.Minimum = 0;
            progressBar1.Maximum = 1;
            progressBar1.Step = 1;
            progressBar1.PerformStep();
 
        }
 
        private void progressBar2_Click(object sender, EventArgs e)
        {
 
        }
 
        private void progressBar1_Click(object sender, EventArgs e)
        {
 
        }
 
        private void timer2_Tick(object sender, EventArgs e)
        {
            progressBar2.Minimum = 0;
            progressBar2.Maximum = 1;
            progressBar2.Step = 1;
            progressBar2.PerformStep();
        }
 
        private void menuStrip1_ItemClicked(object sender, ToolStripItemClickedEventArgs e)
        {
 
        }
 
        private void openToolStripMenuItem_Click(object sender, EventArgs e)
        {
 
        }
 
        private void информацияToolStripMenuItem_Click(object sender, EventArgs e)
        {
            MessageBox.Show("Сообщение");
        }
 
        private void menuStrip2_ItemClicked(object sender, ToolStripItemClickedEventArgs e)
        {
 
        }
 
    }
}