Applying the character of rotation matrices, it is not difficult to obtain the recurrence formulas of direction cosines of Cartesian unit vectors, to calculate the scalar products and triple products of these unit vectors, and to derive the 6th constraint equation. 文中利用旋转矩阵的性质,容易得出各坐标轴单位矢量的方向余弦的递推公式,求出这些单位矢量的标积和混合积,并且可以方便地导出第六个约束方程,推导颇为简捷。