二维空间中的多边形逼近。确定三个顶点a，b，c并且其函数值按从小到大排列。将二维空间中的多边形三个顶点的函数值按从小到大顺序排列， 判断三点或三点函数值的距离是否小于给定阈值.若小于阈值停止循环，得最优解x0=a-Two-dimensional space polygon approximation. Determine the three vertices a, b, c and its function value in ascending order. The three-dimensio