Given a circle centre O and radius r in , the inversion in this circle is the mapping defined by , where lies on the straight line through O and A, and on the same side of O as A, and . It will be investigated the property of inversion related to four harmonic points. The result is that the cross-ratio of any four coplanar points A, B, C, D is invariant under inversion. Hence, the inversion preserves the four harmonic points.
Keywords : inversion, cross ratio, four harmonic points.

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