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Two Graphs of linear equations in two variables

In mathematics, a linear equation is an equation that may be put in the form

where are the variables (or unknowns or indeterminates), and 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 coefficient s are required to not be all zero.

In other words, a linear equation is obtained by equating to zero a linear polynomial over some field, from which the coefficients are taken (the symbols used for the variables are supposed to not denote any element of the field).

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 . 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 n variables form a hyperplane (a subspace of dimension n – 1) in the Euclidean space of dimension n.

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.

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One variableEdit

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 a ≠ 0. In this case, the name unknown is sensibly given to the variable x.

If a = 0, there are two cases. Either b equals also 0, and every number is a solution. Otherwise b ≠ 0, and there is no solution. In this latter case, the equation is said to be inconsistent.

Two variablesEdit

In the case of two variables, any linear equation can be put in the form

 

where the variables are x and y, and the coefficients are a, b and c.

An equivalent equation (that is an equation with exactly the same solutions) is

 

with A = a, B = b, and C = –c

These equivalent variants are sometimes given generic names, such as general form or standard form.[1]

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 functionEdit

If b ≠ 0, the equation

 

is a linear equation in the single variable y for every value of x. It has therefore a unique solution for y, which is given by

 

This defines a function. The graph of this function is a line with slope   and y-intercept   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 c = 0, 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 interpretationEdit

 
Vertical line of equation x = a
 
Horizontal line of equation y = b

Each solution (x, y) 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 a and b 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 b ≠ 0, the line is the graph of the function of x that has been defined in the preceding section. If b = 0, the line is a vertical line (that is a line parallel to the y-axis) of equation   which is not the graph of a function of x.

Similarly, if a ≠ 0, the line is the graph of a function of y, and, if a = 0, one has a horizontal line of equation  

Equation of a lineEdit

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 formEdit

A non-vertical line can be defined by its slope (mathematics) m, and its y-intercept y0 (the y coordinate of its intersection with the y-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 x-intercept x0. In this case, its equation can be written

 

or, equivalently,

 

These form rely on the habit of considering a non vertical line as the graph of a function.[2] For a line given by an equation

 

these forms can be easily deduced from the relations

 

Point–slope formEdit

A non-vertical line can be defined by its slope (mathematics) m, and the coordinates   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 formEdit

A line that is not parallel to an axis and does not passes through origin cuts the axes in two different points. The intercept values x0 and y0 of these two points are nonzero, and a equation of the line is[3]

 

(It easy to verify that the line defined by this equation has x0 and y0 as intercept values).

Two-point formEdit

Given two different points (x1, y1 and (x2, y2, there is exactly one line that passes through them. There are several ways to write a linear equation of is line.

If x1x2, the slope of the line is   Thus, a point-slope form is[3]

 

By clearing denominators, one gets the equation

 

which is valid also when x1 = x2 (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 formEdit

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   is the result of expanding the determinant in the equation

 

The equation   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 n points in a space of dimension n – 1. These equations rely on the condition of linear dependence of points in a projective space.

More than two variablesEdit

A linear equation with more than two variables may always be assumed to have the form

 

The coefficient b, often denoted a0 is called the constant term, sometimes the absolute term,[citation needed]. Depending of the context, the term coefficient can be reserved for the ai with i > 0.

When dealing with   variables, it is common to use   and   instead of indexed variables.

A solution of such an equation is a n-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 b ≠ 0) as having no solution, or all n-tuples are solutions.

The n-tuples that are solutions of a linear equation in n variables are the Cartesian coordinates of the points of an (n – 1-dimensional hyperplane in an n-dimensional 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 aj ≠ 0, then the equation can be solved for xj, yelding

 

If the coefficients are real numbers, this defines a real-valued function of n real variables.

See alsoEdit

NotesEdit

ReferencesEdit

  • Barnett, R.A.; Ziegler, M.R.; Byleen, K.E. (2008), College Mathematics for Business, Economics, Life Sciences and the Social Sciences (11th ed.), Upper Saddle River, N.J.: Pearson, ISBN 0-13-157225-3
  • Larson, Ron; Hostetler, Robert (2007), Precalculus:A Concise Course, Houghton Mifflin, ISBN 978-0-618-62719-6
  • Wilson, W.A.; Tracey, J.I. (1925), Analytic Geometry (revised ed.), D.C. Heath

External linksEdit