Coplanarity

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In geometry, a set of points in space is coplanar if the points all lie in the same geometric plane. For example, three points are always coplanar; but four points in space are usually not coplanar.

Points can be shown to be coplanar by determining that the scalar product of a vector that is normal to the plane and a vector from any point on the plane to the point being tested is 0.

Distance geometry provides a solution to the problem of determining if a set of points is coplanar, knowing only the distances between them.

[edit] Properties

If three 3-dimensional vectors \mathbf{a}, \mathbf{b} and \mathbf{c} are coplanar, and \mathbf{a}\cdot\mathbf{b} = 0, then

(\mathbf{c}\cdot\mathbf{\hat a})\cdot\mathbf{\hat a} + (\mathbf{c}\cdot\mathbf{\hat b})\cdot\mathbf{\hat b}  = \mathbf{\hat c},

where \mathbf{\hat a} denotes the unit vector in the direction of \mathbf{a}.

Or, the vector resolutes of \mathbf{c} on \mathbf{a} and \mathbf{c} on \mathbf{b} add to give the original \mathbf{c}.


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