摘要: 有用的公式：\mathop A\limits^ \to \times (\mathop B\limits^ \to \times \mathop C\limits^ \to ) = (\mathop A\limits^ \to \bullet \mathop C\limits^ \to )\mathop B\limits^ \to - (\mathop A\limits^ \to \bullet \mathop B\limits^ \to )\mathop C\limits^ \to