命题设凸四边形ABCD的对角线AC、BD的交点为M，过点M作AD的平行线分别交AB、CD于点E、F，交BC的延长线于点O．P是以O为圆山，OM为半径的圆上一点．求证：∠OPF＝∠OEP（图1）．这是1996年全国初中数学联赛第二试的第二题．事实上，命题的结论并非局限在凸四边形中，倘若将题设 The proposition is to set the intersection of the diagonal AC and BD of the convex quadrangle ABCD to be M, and the parallel lines passing over the point M for AD to AB and CD respectively at points E and F, and the extension line of BC to point O. P is a point on the circle where O is the radius and OM is the radius. Verification: ∠OPF=∠OEP (Figure 1). This is the second question of the second examination of the National Junior High School Mathematics League in 1996. In fact, the conclusion of the proposition is not limited to the convex quadrilateral, if