双重星系或天体系统，用动力学方法测定其质量时，观测到的视向速度和距离间距，都是各自值的投影值，可用统计方法，求出一个关于它们的平均投影因子。

Both of the observed radial velocity and separation being projective values, in order to determine the mass of binary galaxies the average projection factor may be derived by a statistical method.

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