public class Trigon {

 public static void main(String[] args) {
        double x1 = 9;
        double y1 = 9;
        double x2 = 4;
        double y2 = 4;
  
  Point sp = new Point(x1,y1);//sp是直線上的一點
  Point tp = new Point(x2,y2);//tp是直線上的另一點，也是三角形的頂點
  double arrowWidth = 3;   //設置三角形的半邊底長
  double arrowHeight = 6; //設置三角形的高

  double angle = Math.atan2(tp.y-sp.y, tp.x-sp.x); //求已知直線的傾斜角
  
  System.out.println("該直線的傾斜角度是：" + angle);

  double point_x1 = tp.x-arrowHeight*Math.cos(angle)-arrowWidth*Math.sin(angle);
  double point_y1 = tp.y-arrowHeight*Math.sin(angle)+arrowWidth*Math.cos(angle);

  double point_x2 =tp.x-arrowHeight*Math.cos(angle)+arrowWidth*Math.sin(angle);
  double point_y2 = tp.y-arrowHeight*Math.sin(angle)-arrowWidth*Math.cos(angle);
  
  Point p1 = new Point(point_x1,point_y1);
  Point p2 = new Point(point_x2,point_y2);
  p1.out();
  p2.out();
 }
}
//定義定點類
class Point{
 double x;
 double y;
 
 public Point(){}
 public Point(double x, double y){
  this.x = x;
  this.y = y;
 }
 
 public void out(){
  System.out.println("該頂點的X坐標：" + x);
  System.out.println("該頂點的Y坐標：" + y);
 }
}

參考：http://blog.csdn.net/james999/archive/2008/09/05/2885380.aspx