用第归计算多边形的面积,如果是四边形,就把它切成两个三角形,总之,只计算三角形的面积,然后把所有三角形的面积加起来就是总面积-The area of a polygon in Cartesian space is to be calculated. Each vertex of the polygon is at a coordinate (x,y). A polygon can be divided in half creating 2 sub polygons by adding a