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{{incomplete|the case of multivariate functions and vector valued functions, which must be considered, as this article is linked to from [[Jacobian matrix]]|date=February 2020}}
[[Image:wiki linear function.png|thumb|right|Graph of the linear function: <math>y(x) = -x + 2</math>]]
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If <math>b=0</math> then the linear function is said to be ''homogeneous''. Such function defines a line that passes through the origin of the coordinate system, that is, the point <math>(x,y)=(0,0)</math>. In advanced mathematics texts, the term ''linear function'' often denotes specifically homogeneous linear functions, while the term [[affine function]] is used for the general case, which includes <math>b\neq0</math>.
The natural [[Domain of a function|___domain]] of a linear function <math>f(x)</math>, the set of allowed input values for {{math|''x''}}, is the entire set of [[real number]]s, <math>x\in \mathbb R.</math> One can also consider such functions with {{math|''x''}} in an arbitrary [[field (mathematics)|field]], taking the coefficients {{math|''a, b''}} in that field.
The graph <math>y=f(x)=ax+b</math> is a non-vertical line having exactly one intersection with the {{math|''y''}}-axis, its {{math|''y''}}-intercept point <math>(x,y)=(0,b).</math> The {{math|''y''}}-intercept value <math>y=f(0)=b</math> is also called the ''initial value'' of <math>f(x).</math> If <math>a\neq 0,</math> the graph is a non-horizontal line having exactly one intersection with the {{math|''x''}}-axis, the {{math|''x''}}-intercept point <math>(x,y)=(-\tfrac ba,0).</math> The {{math|''x''}}-intercept value <math>x=-\tfrac ba,</math> the solution of the equation <math>f(x)=0,</math> is also called the ''root'' or [[zero of a function|''zero'']] of <math>f(x).</math>
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==Slope==
[[File:Slope picture.svg|thumb|right|128px|The slope of a line is the ratio <math>\tfrac{\Delta y}{\Delta x}</math> between a change in {{mvar|x}}, denoted <math>\Delta x</math>, and the corresponding change in {{mvar|y}}, denoted <math>\Delta y</math>]]
The [[slope (mathematics)|slope]] of a nonvertical line is a number that measures how steeply the line is slanted (rise-over-run). If the line is the graph of the linear function <math>f(x) = ax + b</math>, this slope is given by the constant {{mvar|a}}.
The slope measures the constant rate of change of <math>f(x)</math> per unit change in ''x'': whenever the input {{mvar|x}} is increased by one unit, the output changes by {{mvar|a}} units: <math>f(x{+}1)=f(x)+a</math>, and more generally <math>f(x{+}\Delta x)=f(x)+a\Delta x</math> for any number <math>\Delta x</math>. If the slope is positive, <math>a > 0</math>, then the function <math>f(x)</math> is increasing; if <math>a < 0</math>, then <math>f(x)</math> is decreasing
In [[differential calculus|calculus]], the derivative of a general function measures its rate of change. A linear function <math>f(x)=ax+b</math> has a constant rate of change equal to its slope {{mvar|a}}, so its derivative is the constant function <math>f\,'(x)=a</math>.
The fundamental idea of differential calculus is that any [[differentiable function|smooth]] function <math>f(x)</math> (not necessarily linear) can be closely [[linear approximation|approximated]] near a given point <math>x=c</math> by a unique linear function. The [[derivative]] <math>f\,'(c)</math> is the slope of this linear function, and the approximation is: <math>f(x) \approx f\,'(c)(x{-}c)+f(c)</math> for <math>x\approx c</math>. The graph of the linear approximation is the [[tangent line]] of the graph <math>y=f(x)</math> at the point <math>(c,f(c))</math>. The derivative slope <math>f\,'(c)</math> generally varies with the point ''c''. Linear functions can be characterized as the only real functions whose derivative is constant: if <math>f\,'(x)=a</math> for all ''x'', then <math>f(x)=ax+b</math> for <math>b=f(0)</math>.
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A given linear function <math>f(x)</math> can be written in several standard formulas displaying its various properties. The simplest is the ''slope-intercept form'':
:<math>f(x)= ax+b</math>,
from which one can immediately see the slope ''a'' and the initial value <math>f(0)=b</math>, which is the ''y''-intercept of the graph <math>y=f(x)</math>.
Given a slope ''a'' and one known value <math>f(x_0)=y_0</math>, we write the ''point-slope form'':
:<math>f(x) = a(x{-}x_0)+y_0</math>.
In graphical terms, this gives the line <math>y=f(x)</math> with slope ''a'' passing through the point <math>(x_0,y_0)</math>.
The ''two-point form'' starts with two known values <math>f(x_0)=y_0</math> and <math>f(x_1)=y_1</math>. One computes the slope <math>a=\tfrac{y_1-y_0}{x_1-x_0}</math> and inserts this into the point-slope form:
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===Example===
Suppose salami and sausage cost €6 and €3 per kilogram, and we wish to buy €12 worth. How much of each can we purchase? If ''x'' kilograms of salami and ''y'' kilograms of sausage costs a total of €12 then, €6×''x'' + €3×''y'' = €12. Solving for ''y'' gives the point-slope form <math>y = -2x + 4</math>, as above. That is, if we first choose the amount of salami ''x'', the amount of sausage can be computed as a function <math>y = f(x) = -2x + 4</math>. Since salami costs twice as much as sausage, adding one kilo of salami decreases the sausage by 2 kilos: <math>f(x{+}1) = f(x) - 2</math>, and the slope is −2. The ''y''-intercept point <math>(x,y)=(0,4)</math> corresponds to buying only
Note that the graph includes points with negative values of ''x'' or ''y'', which have no meaning in terms of the original variables (unless we imagine selling meat to the butcher). Thus we should restrict our function <math>f(x)</math> to the ___domain <math>0\le x\le 2</math>.
Also, we could choose ''y'' as the independent variable, and compute ''x'' by the [[inverse function|inverse]] linear function: <math>x = g(y) = -\tfrac12 y +2</math> over the ___domain <math>0\le y \le 4</math>.
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:<math>r =f(\theta ) = a\theta + b</math>
is an [[Archimedean spiral]] if <math>a \neq 0</math> and a [[circle]] otherwise.
== Notes ==▼
== See also ==
* [[Affine map]], a generalization
* [[Arithmetic progression]], a linear function of integer argument
▲== Notes ==
{{Reflist}}
== References ==
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{{Polynomials}}
{{Authority control}}
[[Category:Calculus]]
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