说明：  平面应力矩形单元有限元分析，并画出网格图形，用星号标记受力点，用叉号标记受约束点，最后求出力、位移和主应力。
(Rectangular plane stress finite element analysis, and draw the grid graph, marked with an asterisk stress points marked with crosses by constraints, and finally seek to contribute, displacement and principal stress.)