$$\begin{array}{l}\mathrm{一个}＝{\mathrm{一个}}_1\stackrel{＾}{我}+{\mathrm{一个}}_2\stackrel{＾}{j}+{\mathrm{一个}}_{3.}\stackrel{＾}{k}，\\ B＝b_1\stackrel{＾}{我}+b_2\stackrel{＾}{j}+b_{3.}\stackrel{＾}{k}．\end{array}$$

$$\begin{array}{l}C＝\mathrm{一个}\times B＝\left|\begin{array}{cc}\begin{array}{cc}{\stackrel{＾}{我}}_{}& {\stackrel{＾}{j}}_{}\end{array}& {\stackrel{＾}{k}}_{}\\ \begin{array}{cc}\begin{array}{c}{\mathrm{一个}}_1\\ b_1\end{array}& \begin{array}{c}{\mathrm{一个}}_2\\ b_2\end{array}\end{array}& \begin{array}{c}{\mathrm{一个}}_{3.}\\ b_{3.}\end{array}\end{array}\right|\\ \\ ＝（{\mathrm{一个}}_2{b}_{3.}-{\mathrm{一个}}_{3.}{b}_2）\stackrel{＾}{我}+（{\mathrm{一个}}_{3.}{b}_1-{\mathrm{一个}}_1{b}_{3.}）\stackrel{＾}{j}+（{\mathrm{一个}}_1{b}_2-{\mathrm{一个}}_2{b}_1）\stackrel{＾}{k}．\end{array}$$

$$\Vert \mathrm{一个}\times B\Vert ＝\Vert \mathrm{一个}\Vert \Vert B\Vert 罪\mathrm{\alpha .}．$$