为方便,下面#后的代表向量.#CD=#BD-#BC,#AC=#BC-#BA,#AD=#BD-#BA.对角线的点积:#AC·#BD=(#BC-#BA)·#BD=#BC·#BD-#BA·#BD 两组对边平方和分别为:AB2+CD2=AB2+(#BD-#BC)2=AB2+BD2+BC2-2#BD·#BC AD2+BC2=(#BD-#BA)2...