Linear functions
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Linear Functions. Definition. A function f is linear if its domain is a set of numbers and it can be expressed in the form where m and b are constants and x denotes an arbitrary element of the domain of f . Change and Rate of Change. Definition.

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Definition
Definition

A function f is linear if its domain is a set of numbers and it can be expressed in the form

where m and b are constants and x denotes an arbitrary element of the domain of f.



Definition1
Definition

  • If x1 and x2 are distinct members of the domain of f, the change in f from x1 to x2 is f(x2) – f(x1). The rate of change of f over the interval from x1 to x2 is


Notation
Notation

  • Let Dx = x2 – x1 denote the change in x. Let Df =f(x2) – f(x1) denote the change in f.

  • The rate of change is the ratio


Exercise
Exercise

  • For real numbers x, let . Find the change in f from x1 = 1 to x2 = 4.

  • Find the rate of change of f over the interval from 0 to 3 .

  • Find a general formula for the rate of change over the interval from x1 to x2 for any x1 and x2.


A characterization of linear functions
A Characterization of Linear Functions

A function from the real numbers to the real numbers is linear if and only if its rate of change is the same for all intervals. If so, the rate of change is the constant m in the formula



Two distinct points

in the plane determine one and only one straight line


Point slope form
Point-Slope Form

Let be two distinct points in the plane.

Case 1:

Set

(slope)

Equation:

or


Case 2:

Equation: x = c.


Point slope form1
Point-Slope Form

Suppose it is known that a line passes through the point with coordinates and that it has slope m. Then the equation of the line is


Slope intercept form
Slope Intercept Form

  • y = f(x) = mx + b

  • m = rate of change of f = slope of the line = tangent of angle between the x-axis and the line

  • b = f(0) = y-intercept of the line



The symmetric form
The Symmetric Form

  • Slope-intercept and point-slope forms cannot handle vertical lines in the xy plane.

  • Symmetric form does not select one variable as the independent variable and the other as the dependent variable. c, d, and e are constants.


Exercise1
Exercise

The graph of a linear function is the line whose equation is

What is the rate of change of f? What are f(0) and f(-2)?



General form of a linear system of two equations in two unknowns
General Form of a Linear System of Two Equations in Two Unknowns

Equations in Symmetric

Form of Two Straight Lines


Three possibilities for solutions
Three Possibilities for Solutions Unknowns

  • The lines are not parallel and intersect in one and only one point. That is, there is one and only one solution of the system.

  • The lines are distinct but parallel and do not intersect. There are no solutions.

  • The equations represent the same straight line. There are infinitely many solutions, one for each point on the line.


Examples
Examples: Unknowns

1.

2.

3.




Relationship of the determinant to the question of solutions
Relationship of the Determinant to the Question of Solutions Unknowns

The linear system has a unique solution if and only if the determinant is different from zero.


Cramer s rule
Cramer’s Rule Unknowns

Not necessarily the best

method of solution.


Exercise2
Exercise Unknowns

  • Solve

  • Answer: x=3/7, y=2/7



Example
Example Unknowns

Given y, solve for x:


Example continued
Example (continued) Unknowns

The equation

defines x as a linear function of y. This function is called the inverse of the original function. We write


Equivalence
Equivalence Unknowns

The two equations

and

are equivalent. One is satisfied by a pair (x,y) if and only if the other is.


General expression for the inverse function
General Expression for the Inverse Function Unknowns

  • If f (x) = mx + b and m≠0, then

  • Note: The slope of the inverse function is the reciprocal of the slope of the original function.



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