?using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;

namespace first1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        static double PI = 3.14159;
        double CircleArea(double r)
        {
            return PI * r * r;
        }
        private void Form1_Load(object sender, EventArgs e)
        {

        }

        private void button1_Click(object sender, EventArgs e)
        {
            double radius;                                         //記錄輸入圓的的半徑
            double area;                                           //返回圓的面積
            radius = Convert.ToInt32(TXT1.Text);
            area=CircleArea(radius);
            TXT2.Text = Convert.ToString(area);
        }

        private void button2_Click(object sender, EventArgs e)
        {
            TXT1.Text = "";
            TXT2.Text = "";
        }
    }
}