对于任意四边形ABCD，其对角线AC与BD的中点分别是M、N，AB、CD的延长线交于X。编制C++程序以验证三角形XMN的面积是四边形ABCD面积的1/4。-For arbitrary quadrilateral of ABCD its diagonals AC and BD, the midpoint is M, N, AB, CD extension cable cross-X. The prepare C++ procedures to verify that the area of ​ ​ the triangle XMN 1/4 of the area of ​ ​ quadrilateral ABCD.