In
geometry
Geometry (; ) is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space such as the distance, shape, size, and relative position of figures. A mathematician who works in the field of geometry is c ...
, a set of points in space are coplanar if there exists a geometric
plane that contains them all. For example, three points are always coplanar, and if the points are distinct and
non-collinear, the plane they determine is unique. However, a set of four or more distinct points will, in general, not lie in a single plane.
Two
lines
Line most often refers to:
* Line (geometry), object with zero thickness and curvature that stretches to infinity
* Telephone line, a single-user circuit on a telephone communication system
Line, lines, The Line, or LINE may also refer to:
Ar ...
in three-dimensional space are coplanar if there is a plane that includes them both. This occurs if the lines are
parallel, or if they
intersect each other. Two lines that are not coplanar are called
skew lines
In three-dimensional geometry, skew lines are two lines that do not intersect and are not parallel. A simple example of a pair of skew lines is the pair of lines through opposite edges of a regular tetrahedron. Two lines that both lie in the ...
.
Distance geometry provides a solution technique for the problem of determining whether a set of points is coplanar, knowing only the distances between them.
Properties in three dimensions
In three-dimensional space, two
linearly independent
In the theory of vector spaces, a set of vectors is said to be if there is a nontrivial linear combination of the vectors that equals the zero vector. If no such linear combination exists, then the vectors are said to be . These concepts a ...
vectors with the same initial point determine a plane through that point. Their
cross product
In mathematics, the cross product or vector product (occasionally directed area product, to emphasize its geometric significance) is a binary operation on two vectors in a three-dimensional oriented Euclidean vector space (named here E), and is d ...
is a
normal vector to that plane, and any vector
orthogonal to this cross product through the initial point will lie in the plane.
This leads to the following coplanarity test using a
scalar triple product:
Four distinct points, , are coplanar if and only if,
:
which is also equivalent to
:
If three vectors are coplanar, then if (i.e., and are orthogonal) then
:
where denotes the
unit vector
In mathematics, a unit vector in a normed vector space is a vector (often a spatial vector) of length 1. A unit vector is often denoted by a lowercase letter with a circumflex, or "hat", as in \hat (pronounced "v-hat").
The term ''direction vec ...
in the direction of . That is, the
vector projection
The vector projection of a vector on (or onto) a nonzero vector , sometimes denoted \operatorname_\mathbf \mathbf (also known as the vector component or vector resolution of in the direction of ), is the orthogonal projection of onto a stra ...
s of on and on add to give the original .
Coplanarity of points in ''n'' dimensions whose coordinates are given
Since three or fewer points are always coplanar, the problem of determining when a set of points are coplanar is generally of interest only when there are at least four points involved. In the case that there are exactly four points, several ''ad hoc'' methods can be employed, but a general method that works for any number of points uses vector methods and the property that a plane is determined by two
linearly independent vectors
In the theory of vector spaces, a set of vectors is said to be if there is a nontrivial linear combination of the vectors that equals the zero vector. If no such linear combination exists, then the vectors are said to be . These concepts ar ...
.
In an -dimensional space where , a set of points
are coplanar if and only if the matrix of their relative differences, that is, the matrix whose columns (or rows) are the vectors
is of
rank 2 or less.
For example, given four points
:
if the
matrix
Matrix most commonly refers to:
* ''The Matrix'' (franchise), an American media franchise
** ''The Matrix'', a 1999 science-fiction action film
** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchis ...
:
is of rank 2 or less, the four points are coplanar.
In the special case of a plane that contains the origin, the property can be simplified in the following way:
A set of points and the origin are coplanar if and only if the matrix of the coordinates of the points is of rank 2 or less.
Geometric shapes
A
skew polygon
Skew may refer to:
In mathematics
* Skew lines, neither parallel nor intersecting.
* Skew normal distribution, a probability distribution
* Skew field or division ring
* Skew-Hermitian matrix
* Skew lattice
* Skew polygon, whose vertices do no ...
is a
polygon
In geometry, a polygon () is a plane figure that is described by a finite number of straight line segments connected to form a closed '' polygonal chain'' (or ''polygonal circuit''). The bounded plane region, the bounding circuit, or the two t ...
whose
vertices are not coplanar. Such a polygon must have at least four vertices; there are no skew triangles.
A
polyhedron
In geometry, a polyhedron (plural polyhedra or polyhedrons; ) is a three-dimensional shape with flat polygonal faces, straight edges and sharp corners or vertices.
A convex polyhedron is the convex hull of finitely many points, not all on ...
that has positive
volume
Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). The ...
has vertices that are not all coplanar.
See also
*
Collinearity
*
Plane of incidence
In describing reflection and refraction in optics, the plane of incidence (also called the incidence plane or the meridional plane) is the plane which contains the surface normal and the propagation vector of the incoming radiation. (In wave op ...
References
External links
* {{MathWorld , urlname=Coplanar , title=Coplanar
Geometry