任意给定顶点个数　ｎ，以及顶点坐标　Ｐ（ｘ，ｙ），顶点坐标顺序可以是任意顺序；可以判断出给的是凸多边形或是凹多边形，然后算出多边形的面积；(Any given vertex number n, vertex coordinates P (x, y), vertex coordinates can be in any order, can be determined to give a convex polygon or a concave polygon, and then calculate the area of the polygon;)