求正多边形面积

[java]代码库

package homework;

import java.nio.channels.ScatteringByteChannel;
import java.security.PublicKey;
import java.util.Scanner;

class polygon {
        private double a, n;

        polygon(double an, double nn) {
            a = an;
            n = nn;
        }

        private double angle() {
            return (((n-2)*180)/n)/2;
        }

        private double trangle() {
            double a1=a/2;
            double c = a1* Math.tan(angle()*(Math.PI/180));
            return (float)c*a*0.5;
        }

        double area() {
            return n * trangle();
        }
    }
class main2 {
    public static void main(String[] args) {
        System.out.println("请输入边长和有几条边");
        Scanner reader = new Scanner(System.in);
        double a =reader.nextDouble();
        System.out.println("边长为 "+a+"cm");
        double n = reader.nextDouble();
        System.out.println("边有 "+n+"条");
        if (a<=0 || n<3)
            System.out.println("输入错误");
        polygon s = new polygon(a, n);

        System.out.println("面积为"+s.area()+"cm^2");
    }
}

[代码运行效果截图]