Linear equation

If b ≠ 0, the equation

${\displaystyle ax+by+c=0}$

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

${\displaystyle y=-{\frac {a}{b}}x-{\frac {c}{b}}.}$

This defines a function. The graph of this function is a line with slope ${\displaystyle -{\frac {a}{b}}}$ and y-intercept ${\displaystyle -{\frac {c}{b}}.}$ 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, and the linear functions such that c = 0 are often called linear maps.

Vertical line of equation x = a

Horizontal line of equation y = b

Each solution (x, y) of a linear equation

${\displaystyle ax+by+c=0}$

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 ${\displaystyle x=-{\frac {c}{a}},}$ 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 ${\displaystyle y=-{\frac {c}{b}}.}$

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

A non-vertical line can be defined by its slope 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

${\displaystyle y=mx+y_{0}.}$

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

${\displaystyle y=m(x-x_{0}),}$

or, equivalently,

${\displaystyle y=mx-mx_{0}.}$

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

${\displaystyle ax+by+c=0,}$

these forms can be easily deduced from the relations

${\displaystyle {\begin{aligned}m&=-{\frac {a}{b}},\\x_{0}&=-{\frac {c}{a}},\\y_{0}&=-{\frac {c}{b}}.\end{aligned}}}$

A non-vertical line can be defined by its slope m, and the coordinates ${\displaystyle x_{1},y_{1}}$ of any point of the line. In this case, a linear equation of the line is

${\displaystyle y=y_{1}+m(x-x_{1}),}$

or

${\displaystyle y=mx+y_{1}-mx_{1}.}$

This equation can also be written

${\displaystyle y-y_{1}=m(x-x_{1})}$

for emphasizing that the slope of a line can be computed from the coordinates of any two points.

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 x₀ and y₀ of these two points are nonzero, and an equation of the line is[3]

${\displaystyle {\frac {x}{x_{0}}}+{\frac {y}{y_{0}}}=1.}$

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

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 this line.

If x₁ ≠ x₂, the slope of the line is ${\displaystyle {\frac {y_{2}-y_{1}}{x_{2}-x_{1}}}.}$ Thus, a point-slope form is[3]

${\displaystyle y-y_{1}={\frac {y_{2}-y_{1}}{x_{2}-x_{1}}}(x-x_{1}).}$

By clearing denominators, one gets the equation

${\displaystyle (x_{2}-x_{1})(y-y_{1})-(y_{2}-y_{1})(x-x_{1})=0,}$

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:

${\displaystyle (y_{1}-y_{2})x+(x_{2}-x_{1})y+(x_{1}y_{2}-x_{2}y_{1})=0}$

(exchanging the two points changes the sign of the left-hand side of the equation).

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 ${\displaystyle (x_{2}-x_{1})(y-y_{1})-(y_{2}-y_{1})(x-x_{1})=0}$ is the result of expanding the determinant in the equation

${\displaystyle {\begin{vmatrix}x-x_{1}&y-y_{1}\\x_{2}-x_{1}&y_{2}-y_{1}\end{vmatrix}}=0.}$

The equation ${\displaystyle (y_{1}-y_{2})x+(x_{2}-x_{1})y+(x_{1}y_{2}-x_{2}y_{1})=0}$ can be obtained be expanding with respect to its first row the determinant in the equation

${\displaystyle {\begin{vmatrix}x&y&1\\x_{1}&y_{1}&1\\x_{2}&y_{2}&1\end{vmatrix}}=0.}$

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