Linear equation

In mathematics, a linear equation is an equation that may be put in the form
where formula_2 are the variables (or unknowns), and formula_3 are the coefficients, which are often real numbers. The coefficients may be considered as parameters of the equation, and may be arbitrary expressions, provided they do not contain any of the variables. To yield a meaningful equation, the coefficients formula_4 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 formula_5). 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 of a point of the Euclidean plane. 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 of dimension .
Linear equations occur frequently in all mathematics and their applications in physics and engineering, 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 numbers, 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.
Frequently the term "linear equation" refers implicitly to the case of just one variable.
In this case, the equation can be put in the form
and it has a unique solution 
in the general case where .
In this case, the name "unknown" is sensibly given to the variable .
If , there are two cases. Either equals also 0, and every number is a solution. Otherwise , and there is no solution. In this latter case, the equation is said to be inconsistent.
Two variables.
In the case of two variables, any linear equation can be put in the form
where the variables are and , and the coefficients are , and .
An equivalent equation (that is an equation with exactly the same solutions) is 
with , and 
These equivalent variants are sometimes given generic names, such as "general form" or "standard form".
There are other forms for a linear equation (see below), which can all be transformed in the standard form with simple algebraic manipulations, such as adding the same quantity to both members of the equation, or multiplying both members by the same nonzero constant.
Linear function.
If , the equation 
is a linear equation in the single variable for every value of . It has therefore a unique solution for , which is given by 
This defines a function. The graph of this function is a line with slope formula_12 and -intercept formula_13 The functions whose graph is a line are generally called "linear functions" in the context of calculus. However, in linear algebra, 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
may be viewed as the Cartesian coordinates of a point in the Euclidean plane. 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 formula_15 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 formula_16
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.
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 
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
or, equivalently,
These forms rely on the habit of considering a non vertical line as the graph of a function. For a line given by an equation 
these forms can be easily deduced from the relations 
Point–slope form.
A non-vertical line can be defined by its slope , and the coordinates formula_22 of any point of the line. In this case, a linear equation of the line is
or 
This equation can also be written
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
(It 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 formula_27 Thus, a point-slope form is
By clearing denominators, one gets the equation 
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:
(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. There are two common ways for that.
The equation formula_31 is the result of expanding the determinant in the equation
The equation formula_33 can be obtained be expanding with respect to its first row the determinant in the equation
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
The coefficient , often denoted is called the "constant term", sometimes the "absolute term". Depending on the context, the term "coefficient" can be reserved for the with .
When dealing with formula_36 variables, it is common to use formula_37 and formula_38 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 of the points of an -dimensional hyperplane in an Euclidean space (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 
If the coefficients are real numbers, this defines a real-valued function of real variables.