Linear equation

A linear equation is an equation involving only the sum of constants or products of constants and the first power of a variable. Such an equation is equivalent to equating a first-degree polynomial to zero. These equations are called "linear" because they represent straight lines in Cartesian coordinates. A common form of a linear equation in two variables is $$y = mx + c$$, (e.g. $$y = 3x + 5$$). In this form, the value $$m$$ will determine the slope or gradient of the line; and the value $$c$$ will determine the point at which the line crosses the y-axis. Equations involving terms such as x2, y1/3, and xy are "non-linear".

Examples of linear equations in two variables:


 * $$3x + 2y = 10\,$$


 * $$3a + 472b = 10b + 37\,$$


 * $$3x + y -5 = -7x + 4y +3\,.$$

Forms of a linear equation
Complicated linear equations, such as the ones above, can be rewritten using the laws of elementary algebra into several simpler forms. In what follows, capital letters represent constants (unspecified but fixed numbers), while x and y are the variables.


 * General form:
 * $$Ax + By + C = 0\,$$
 * Here A and B are not both equal to zero. The equation is usually written so that A &ge; 0, by convention. The graph of the equation is a straight line, and every straight line can be represented by an equation in the above form. If A is nonzero, then the x-intercept, that is the x-coordinate of the point where the graph crosses the x-axis (y is zero), is  &minus;C/A. If B is nonzero, then the y-intercept, that is the y-coordinate of the point where the graph crosses the y-axis (x is zero), is &minus;C/B, and the slope of the line is &minus;A/B.


 * Standard form:
 * $$Ax + By = D\,$$
 * Here A and B are not both equal to zero. As above, usually A &ge; 0. The standard form can be converted to the general form by setting C = &minus;D.


 * Intercept form:
 * $$\frac{x}{E} + \frac{y}{F} = 1$$
 * Here E and F must be nonzero. The graph of the equation has x-intercept E and y-intercept F. The intercept form can be converted to the standard form by setting A = 1/E, B = 1/F and D = 1.


 * Slope-intercept form:
 * $$y = Mx + F\,$$
 * M is the slope of the line and F is the y-intercept.


 * Point-slope form:
 * $$y - K = M(x - H)\,$$
 * The graph passes through the point (H,K) and has slope M.


 * Two-point form:
 * $$y - K = \frac{Q - K}{P - H} (x - H)$$
 * Here P &ne; H. The graph passes through the points (H,K) and (P,Q), and has slope M = (Q&minus;K) / (P&minus;H).


 * Parametric form:
 * $$x = Tt + U\,$$ and $$y = Vt + W\,$$
 * Two simultaneous equations in terms of a variable parameter t, with slope M = V / T, x intercept (VU&minus;WT) / V and y intercept (WT&minus;VU) / T.
 * This can also be related to the two-point form with T = P&minus;H, U = H, V = Q&minus;K, and W = K:
 * $$x = (P - H)t + H\,$$ and $$y = (Q - K)t + K\,$$
 * In this case t varies from 0 at point (H,K) to 1 at point (P,Q), with values of t between 0 and 1 providing interpolation and other values of t providing extrapolation.


 * Special cases:
 * $$y = F\,$$
 * This is a special case of the standard form where A = 0 and B = 1, or of the slope-intercept form where the slope M = 0. The graph is a horizontal line with y-intercept equal to F. There is no x-intercept, unless F = 0, in which case the graph of the line is the x-axis, and so every real number is an x-intercept.


 * $$x = E\,$$
 * This is a special case of the standard form where A = 1 and B = 0. The graph is a vertical line with x-intercept equal to E. The slope is undefined. There is no y-intercept, unless E = 0, in which case the graph of the line is the y-axis, and so every real number is a y-intercept.


 * $$0 = 0\,$$
 * In this case all variables and constants have canceled out, leaving a trivially true statement. The original equation, therefore, would be called an identity and one would not consider the graph (it would be the entire xy-plane). An example is 2x + 4y = 2(x + 2y). The two expressions on either side of the equal sign are always equal, no matter what values are used for x and y.

Note that if algebraic manipulation leads to a statement such as 1 = 0, then the original equation is called inconsistent, meaning it is untrue for any values of x and y. An example would be 3x + 2 = 3x &minus; 5.

In addition, there may be more than two variables in the equation or several simultaneous equations. For more information see System of linear equations.

Connection with linear functions and operators
In all of the named forms above (assuming the graph is not a vertical line), the variable y is a function of x, and the graph of this function is the graph of the equation.

In the particular case that the line crosses through the origin, if the linear equation is written in the form y = f(x) then f has the properties:
 * f(x + y) = f(x) + f(y)
 * f(ax) = af(x)

where a is any scalar. A function which satisfies these properties is called a linear function, or more generally a linear operator.

Because of the linear property above, the solutions of linear equations of this kind can in general be described as a superposition of other solutions of the same equation. This makes linear equations particularly easy to solve and reason about.

Linear equations occur with great regularity in applied mathematics. While they arise quite naturally when modeling many phenomena, they are particularly useful since many non-linear equations may be reduced to linear equations by assuming that quantities of interest vary to only a small extent from some "background" state.