// 个人对本章理解非常差……但是本章非常重要,同志们要好好看啊
import java.util.Scanner;
public class Code_Practice_4 {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.print("Enter the length from the center to a vertex:");
double length = input.nextDouble();
double side = 2 * length * Math.sin(Math.PI / 5);
double area = (5 * side * side) / (4 * Math.tan(Math.PI / 5));
System.out.printf("The area of the pentagon is %.2f:",area);
}
}