Elementary geometry: showing that four points are coplanar – math.stackexchange.com
Kuba
Suppose we are given four coplanar points $A$, $B$, $C$, $D$, in three-dimesional Euclidean space.
Suppose that we are also given four points $A'$, $B'$, $C'$, $D'$, such that $\left| AB \right| = ...