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mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern mathematics ...
, a linear equation is an equation that may be put in the form a_1x_1+\ldots+a_nx_n+b=0, where x_1,\ldots,x_n are the variables (or unknowns), and b,a_1,\ldots,a_n are the coefficients, which are often
real number In mathematics, a real number is a number that can be used to measure a ''continuous'' one-dimensional quantity such as a distance, duration or temperature. Here, ''continuous'' means that values can have arbitrarily small variations. Every ...
s. The coefficients may be considered as
parameter A parameter (), generally, is any characteristic that can help in defining or classifying a particular system (meaning an event, project, object, situation, etc.). That is, a parameter is an element of a system that is useful, or critical, when ...
s of the equation, and may be arbitrary expressions, provided they do not contain any of the variables. To yield a meaningful equation, the coefficients a_1, \ldots, a_n are required to not all be zero. Alternatively, a linear equation can be obtained by equating to zero a linear polynomial over some field, from which the coefficients are taken. The solutions of such an equation are the values that, when substituted for the unknowns, make the equality true. In the case of just one variable, there is exactly one solution (provided that a_1\ne 0). Often, the term ''linear equation'' refers implicitly to this particular case, in which the variable is sensibly called the ''unknown''. In the case of two variables, each solution may be interpreted as the
Cartesian coordinates A Cartesian coordinate system (, ) in a plane is a coordinate system that specifies each point uniquely by a pair of numerical coordinates, which are the signed distances to the point from two fixed perpendicular oriented lines, measured in ...
of a point of the
Euclidean plane In mathematics, the Euclidean plane is a Euclidean space of dimension two. That is, a geometric setting in which two real quantities are required to determine the position of each point ( element of the plane), which includes affine notions ...
. The solutions of a linear equation form a line in the Euclidean plane, and, conversely, every line can be viewed as the set of all solutions of a linear equation in two variables. This is the origin of the term ''linear'' for describing this type of equations. More generally, the solutions of a linear equation in variables form a hyperplane (a subspace of dimension ) in the
Euclidean space Euclidean space is the fundamental space of geometry, intended to represent physical space. Originally, that is, in Euclid's ''Elements'', it was the three-dimensional space of Euclidean geometry, but in modern mathematics there are Euclidea ...
of dimension . Linear equations occur frequently in all mathematics and their applications in
physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which ...
and
engineering Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad range of more speciali ...
, partly because non-linear systems are often well approximated by linear equations. This article considers the case of a single equation with coefficients from the field of
real number In mathematics, a real number is a number that can be used to measure a ''continuous'' one-dimensional quantity such as a distance, duration or temperature. Here, ''continuous'' means that values can have arbitrarily small variations. Every ...
s, for which one studies the real solutions. All of its content applies to complex solutions and, more generally, for linear equations with coefficients and solutions in any field. For the case of several simultaneous linear equations, see system of linear equations.


One variable

A linear equation in one variable is of the form ax+b=0, where and are real numbers and a\neq 0 . The root of x=-\frac ba.


Two variables

A linear equation in two variables and is of the form ax+by+c=0, where , and are real numbers such that a^2+b^2\neq 0 . It has infinitely many possible solutions.


Linear function

If , the equation :ax+by+c=0 is a linear equation in the single variable for every value of . It has therefore a unique solution for , which is given by :y=-\frac ab x-\frac cb. This defines a function. The graph of this function is a line with
slope In mathematics, the slope or gradient of a line is a number that describes both the ''direction'' and the ''steepness'' of the line. Slope is often denoted by the letter ''m''; there is no clear answer to the question why the letter ''m'' is use ...
-\frac ab and -intercept -\frac cb. The functions whose graph is a line are generally called ''linear functions'' in the context of
calculus Calculus, originally called infinitesimal calculus or "the calculus of infinitesimals", is the mathematics, mathematical study of continuous change, in the same way that geometry is the study of shape, and algebra is the study of generalizati ...
. However, in
linear algebra Linear algebra is the branch of mathematics concerning linear equations such as: :a_1x_1+\cdots +a_nx_n=b, linear maps such as: :(x_1, \ldots, x_n) \mapsto a_1x_1+\cdots +a_nx_n, and their representations in vector spaces and through matrice ...
, a linear function is a function that maps a sum to the sum of the images of the summands. So, for this definition, the above function is linear only when , that is when the line passes through the origin. For avoiding confusion, the functions whose graph is an arbitrary line are often called ''affine functions''.


Geometric interpretation

Each solution of a linear equation :ax+by+c=0 may be viewed as the
Cartesian coordinates A Cartesian coordinate system (, ) in a plane is a coordinate system that specifies each point uniquely by a pair of numerical coordinates, which are the signed distances to the point from two fixed perpendicular oriented lines, measured in ...
of a point in the
Euclidean plane In mathematics, the Euclidean plane is a Euclidean space of dimension two. That is, a geometric setting in which two real quantities are required to determine the position of each point ( element of the plane), which includes affine notions ...
. With this interpretation, all solutions of the equation form a line, provided that and are not both zero. Conversely, every line is the set of all solutions of a linear equation. The phrase "linear equation" takes its origin in this correspondence between lines and equations: a ''linear equation'' in two variables is an equation whose solutions form a line. If , the line is the graph of the function of that has been defined in the preceding section. If , the line is a ''vertical line'' (that is a line parallel to the -axis) of equation x=-\frac ca, which is not the graph of a function of . Similarly, if , the line is the graph of a function of , and, if , one has a horizontal line of equation y=-\frac cb.


Equation of a line

There are various ways of defining a line. In the following subsections, a linear equation of the line is given in each case.


Slope–intercept form or Gradient-intercept form

A non-vertical line can be defined by its slope , and its -intercept (the coordinate of its intersection with the -axis). In this case its ''linear equation'' can be written :y=mx+y_0. If, moreover, the line is not horizontal, it can be defined by its slope and its -intercept . In this case, its equation can be written :y=m(x-x_0), or, equivalently, :y=mx-mx_0. These forms rely on the habit of considering a non vertical line as the
graph of a function In mathematics, the graph of a function f is the set of ordered pairs (x, y), where f(x) = y. In the common case where x and f(x) are real numbers, these pairs are Cartesian coordinates of points in two-dimensional space and thus form a subs ...
. For a line given by an equation :ax+by+c = 0, these forms can be easily deduced from the relations :\begin m&=-\frac ab,\\ x_0&=-\frac ca,\\ y_0&=-\frac cb. \end


Point–slope form or Point-gradient form

A non-vertical line can be defined by its slope , and the coordinates x_1, y_1 of any point of the line. In this case, a linear equation of the line is :y=y_1 + m(x-x_1), or :y=mx +y_1-mx_1. This equation can also be written :y-y_1=m(x-x_1) for emphasizing that the slope of a line can be computed from the coordinates of any two points.


Intercept form

A line that is not parallel to an axis and does not pass through the origin cuts the axes in two different points. The intercept values and of these two points are nonzero, and an equation of the line is :\frac + \frac = 1. (It is easy to verify that the line defined by this equation has and as intercept values).


Two-point form

Given two different points and , there is exactly one line that passes through them. There are several ways to write a linear equation of this line. If , the slope of the line is \frac. Thus, a point-slope form is :y - y_1 = \frac (x - x_1). By clearing denominators, one gets the equation : (x_2 - x_1)(y - y_1) - (y_2 - y_1)(x - x_1)=0, which is valid also when (for verifying this, it suffices to verify that the two given points satisfy the equation). This form is not symmetric in the two given points, but a symmetric form can be obtained by regrouping the constant terms: :(y_1-y_2)x + (x_2-x_1)y + (x_1y_2 - x_2y_1) =0 (exchanging the two points changes the sign of the left-hand side of the equation).


Determinant form

The two-point form of the equation of a line can be expressed simply in terms of a
determinant In mathematics, the determinant is a scalar value that is a function of the entries of a square matrix. It characterizes some properties of the matrix and the linear map represented by the matrix. In particular, the determinant is nonzero if a ...
. There are two common ways for that. The equation (x_2 - x_1)(y - y_1) - (y_2 - y_1)(x - x_1)=0 is the result of expanding the determinant in the equation :\beginx-x_1&y-y_1\\x_2-x_1&y_2-y_1\end=0. The equation (y_1-y_2)x + (x_2-x_1)y + (x_1y_2 - x_2y_1)=0 can be obtained be expanding with respect to its first row the determinant in the equation :\begin x&y&1\\ x_1&y_1&1\\ x_2&y_2&1 \end=0. Beside being very simple and mnemonic, this form has the advantage of being a special case of the more general equation of a hyperplane passing through points in a space of dimension . These equations rely on the condition of linear dependence of points in a projective space.


More than two variables

A linear equation with more than two variables may always be assumed to have the form :a_1 x_1 + a_2 x_2 + \cdots + a_n x_n + b=0. The coefficient , often denoted is called the ''constant term'' (sometimes the ''absolute term'' in old booksExtract of page 113
/ref>). Depending on the context, the term ''coefficient'' can be reserved for the with . When dealing with n=3 variables, it is common to use x,\; y and z instead of indexed variables. A solution of such an equation is a -tuples such that substituting each element of the tuple for the corresponding variable transforms the equation into a true equality. For an equation to be meaningful, the coefficient of at least one variable must be non-zero. In fact, if every variable has a zero coefficient, then, as mentioned for one variable, the equation is either ''inconsistent'' (for ) as having no solution, or all are solutions. The -tuples that are solutions of a linear equation in are the
Cartesian coordinates A Cartesian coordinate system (, ) in a plane is a coordinate system that specifies each point uniquely by a pair of numerical coordinates, which are the signed distances to the point from two fixed perpendicular oriented lines, measured in ...
of the points of an -dimensional hyperplane in an
Euclidean space Euclidean space is the fundamental space of geometry, intended to represent physical space. Originally, that is, in Euclid's ''Elements'', it was the three-dimensional space of Euclidean geometry, but in modern mathematics there are Euclidea ...
(or affine space if the coefficients are complex numbers or belong to any field). In the case of three variable, this hyperplane is a plane. If a linear equation is given with , then the equation can be solved for , yielding :x_j = -\frac b -\sum_ \frac x_i . If the coefficients are
real number In mathematics, a real number is a number that can be used to measure a ''continuous'' one-dimensional quantity such as a distance, duration or temperature. Here, ''continuous'' means that values can have arbitrarily small variations. Every ...
s, this defines a real-valued function of real variables.


See also

* Linear equation over a ring * Algebraic equation * Linear inequality * Nonlinear equation


Notes


References

* * *


External links

* {{Authority control Elementary algebra Equations