文[1]介绍了关于四边形的两个定理:中线定理如图凸四边形ABCD中,E,F,G,H是各边中点,EF,GH是两条中线,则2(EF~2-GH~2)=AD~2+BC~2-AB~2-CD~2.对角线定理如图凸四边形ABCD中,对角线AC,BD的夹角为α,α的对应边为AD,BC,则2AC·BDcosα=(AB~2+CD~2-AD~2-BC~2)/2.笔者通过探究,发现在任意的空间(或平面) In [1], two theorems about quadrilateral are introduced. The central theorem is as shown in graph convex quadrilateral ABCD, where E, F, G and H are the midpoints of each edge, EF and GH are two midlines, then 2 GH ~ 2) = AD ~ 2 + BC ~ 2-AB ~ 2-CD ~ 2. Diagonal Theorem As shown in graph convex quadrilateral ABCD, the angle between diagonal AC and BD is α, and the corresponding edge of α is AD , BC, then 2AC · BDcosα = (AB ~ 2 + CD ~ 2-AD ~ 2-BC ~ 2) / 2. The author through exploration and found that in any space (or plane)