若非單位圓時,
設甲、乙中點為M
則\(\overleftrightarrow{OM} = \left [ \begin{array}{ll} x=\frac{ 3}{ 4}t & \\ y= \frac{ 1}{ 4}t &, t \in R \\ z=\frac{ \sqrt[ ]{2 }}{ 4}t \end{array} \right ] \)
丙\(\displaystyle \left ( \frac{ 3}{ 4}t, \frac{ 1}{ 4}t, \frac{ \sqrt{2 } }{ 4}t, \right ) \)
帶入球方程式,解 t 可得丙座標