*2.19（几何：三角形的面积）编写程序，提示用户输入三角形的三个点（x1，y1）、（x2，y2）和（x3，y3），然后显示它的面积。计算三角形面积的公式是：
下面是一个运行示例：
Enter the coordinates of three points separated by spaces\nlike x1 y1 x2 y2 x3 y3 : 1.5 -3.4 4.6 5 9.5 -3.4
The area of the triangle is 33.6
*2.19(Geometry: area of a triangle) Write a program that prompts the user to enter three points, (x1, y1), (x2, y2), and (x3, y3), of a triangle then displays its area. The formula for computing the area of a triangle is
Here is a simple run:
Enter the coordinates of three points separated by spaces\nlike x1 y1 x2 y2 x3 y3 : 1.5 -3.4 4.6 5 9.5 -3.4
The area of the triangle is 33.6
参考代码：
package chapter02; import java.util.Scanner; public class Code_19 { public static void main(String[] args) { double x1, y1; double x2, y2; double x3, y3; //distance between two points double LengthSide1, LengthSide2, LengthSide3; //HalfSumLengthSide represent that half of summation of all of sides double HalfSumLengthSide, AreaTriangle; System.out.print("Enter the coordinates of three points separated" + " by spaces\nlike x1 y1 x2 y2 x3 y3 : "); Scanner PointsInput = new Scanner(System.in); x1 = PointsInput.nextDouble(); y1 = PointsInput.nextDouble(); x2 = PointsInput.nextDouble(); y2 = PointsInput.nextDouble(); x3 = PointsInput.nextDouble(); y3 = PointsInput.nextDouble(); LengthSide1 = Math.pow(Math.pow(x2-x1, 2) + Math.pow(y2-y1, 2), 0.5); LengthSide2 = Math.pow(Math.pow(x3-x1, 2) + Math.pow(y3-y1, 2), 0.5); LengthSide3 = Math.pow(Math.pow(x3-x2, 2) + Math.pow(y3-y2, 2), 0.5); HalfSumLengthSide = (LengthSide1 + LengthSide2 + LengthSide3) / 2; AreaTriangle = Math.pow(HalfSumLengthSide *(HalfSumLengthSide - LengthSide1) *(HalfSumLengthSide - LengthSide2) *(HalfSumLengthSide - LengthSide3), 0.5); System.out.println("The area of the triangle is " + AreaTriangle); PointsInput.close(); } } 结果显示： Enter the coordinates of three points separated by spaces like x1 y1 x2 y2 x3 y3 : 1.5 -3.4 4.6 5 9.5 -3.4 The area of the triangle is 33.600000000000016 Process finished with exit code 0