文[1]、[2]分别介绍了共边定理和共角及其应用,充分体现了这两个定理在平面几何证明中的强大威力.因此,这两个定理值得我们重视和掌握它们的运用技巧.例1如图1,在单等对边四边形ABCD中,AB=CD,E、F分别是AD、BC的中点,直线EF与BA的延长线、CD的延长线分别交于点 In [1], [2], we introduce the co-edge theorems and common angles respectively and their applications, which fully demonstrate the great power of these two theorems in plane geometry proof. Therefore, these two theorems deserve our attention and mastery of them Example 1 As shown in Fig. 1, AB = CD, E and F are the midpoints of AD and BC, respectively, in the single-to-opposite quadrilateral ABCD, and the extension of line EF and BA and the extension of CD are respectively at point