题解 | 三角形面积

import java.util.Scanner;

public class Main {
    static class Point {
        double x, y;
        
        Point(double a, double b) {
            x = a;
            y = b;
        }
    }
    
    static class Triangle {
        Point a, b, c;
        
        Triangle(Point a, Point b, Point c) {
            this.a = a;
            this.b = b;
            this.c = c;
        }
    }
    static double distance(Point a,Point b) {
		double d=Math.sqrt(Math.pow(a.x-b.x, 2)+Math.pow(a.y-b.y, 2));
		return d;
	}
    static double getArea(Triangle T) {
        // TODO: 计算三角形T的面积
        double ba=distance(T.a, T.b);
		 double bc=distance(T.b, T.c);
		 double ac=distance(T.a, T.c);
		 double cosB=(ba*ba+bc*bc-ac*ac)/(2*ba*bc);
		 double sinB=Math.sqrt(1-cosB*cosB);
		 double s=bc*ba*sinB;
	    return s/2;//一定要除二
    }
   
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int x, y;
        
        x = scanner.nextInt();
        y = scanner.nextInt();
        Point a = new Point(x, y);
        
        x = scanner.nextInt();
        y = scanner.nextInt();
        Point b = new Point(x, y);
        
        x = scanner.nextInt();
        y = scanner.nextInt();
        Point c = new Point(x, y);
        
        Triangle T = new Triangle(a, b, c);
        System.out.printf("%.2f%n", getArea(T));
        scanner.close();
    }
}

本题一定要注意三角形面积得除二