A relation is a correspondence between two sets. If x and y are two elements in these sets and if a relation exists between x and y, then x corresponds to y, or y depends on x.

The set of x-coordinates {2,3,4,4,5,6,6,7} corresponds to the set of y coordinates {60,70,70,80,85,85,95,90}

The set of distinct x-coordinates is called the domain of the relation. This is the set of all possible x values specified for a given relation.The set of all distinct y values corresponding to the x-coordinates is called the range.In the example above, Domain = {2,3,4,5,6,7} Range = {60,70,80,85,95,90}

Math 80A Section 3.5 Relations and Functions

Example:

xHours Studying in Math Lab 2 3 4 4 5 6 6 7

y Score on Math Test 60 70 70 80 85 85 95 90

A function, f, is like a machine that receives as input a number, x, from the domain, manipulates it, and outputs the value, y.

The function is simply the process that x goes through to become y. This “machine” has 2 restrictions:

1. It only accepts numbers from the domain of the function.2. For each input, there is exactly one output (which may be repeated for different inputs).

Input x

Output yFunction

f(x)

FUNCTIONS

“OFFICIAL” DEFINITION OF A FUNCTION:Let X and Y be two nonempty sets. A function from X into Y is a relation that associates with each element of X, exactly one element of Y.However, an element of Y may have more than one elements of x associated with it.That is, for each ordered pair (x,y), there is exactly one y value for each x, but there may be multiple x values for each y. The variable x is called the independent variable (also sometimes called the argument of the function), and the variable y is called dependent variable (also sometimes called the image of the function.)

Analogy: In the x-y “relation”-ship, the x’s are the wives and the y’s are the husbands. A husband is allowed to have two or more wives, but each wife(x) is only allowed 1 husband(y).

This relation is not a function because there are two different y-coordinates for the x-coordinate, 4, and also for the x-coordinate, 6.

Example:Given the relation {(-2,-3),(2,3),(-1,2),(-3,4),(3,4)}Is this relation a function?_______________(does any one x-value have more than one corresponding y value?)

What is the domain? -3,-2, -1, 2, 3What is the range? -3, 2, 3, 4

Example:Given the relation {(-4,4),(-2,2),(0,0),(-2,-2)}Is this relation a function?_______________(does any one x-value have more than one corresponding y value?)

What is the domain?___________________What is the range?______________________

A relation is not a function of x if there is more than one corresponding y-value for any x-value.To be a function of x, put the relation in the form y = ___ and there should be only 1 possible solution for any one x-value.

Example 6 p. 202

Are these functions of x?

a) x = y2

y can be or

Not a function of x.

b) y = 2x There is only 1 possibility for any x.

So YES. it is a function of x.

c) x = |y|

Solve for y. x = y or x = -y

y = x or y = -x

Two possibilities, therefore NOT A FUNCTION.

xy

x x

VERTICAL LINE TEST

A graph is the graph of a funciton if and only if there is no vertical line that crosses the graph more than once.

ExampleConsider the equation for the line segment: y = 2x – 5, where the domain is {x|1 ≤ x ≤ 6}Is this equation a function?Notice that for any x, you can only get one answer for y. (E.g. when x =1, then y = 2(1) – 5= -3.) Therefore the equation is a function. Functional notation for this equation would bef(x) = 2x – 5Just replace the y with f(x). Note: This is stated “f of x”,It does not mean f times x, though it looks like that.What is the range?Since this is a straight line, we need only check y values at endpoints of domain. The y values do not fluctuate between these endpoints. The y values are also called function values, so they are often referred to as f(x), which means the value of the function at x (not f times x).The endpoints of the domain are 1 and 6.f(1) = the value of the function (what is y?) when x = 2 = 2(1) – 5 = -3f(6) = the value of the function (what is y?) when x = 6 = 2(6) – 5 = 7So the range is {y|-3 ≤ y ≤ 7}

ExampleFind the range of the function given by the equation f(x) = -3x + 2 if the domain is {-4,-2,0,2,4}.Recall domain means possible x-values.

The ordered pairs that belong to thisfunction are graphed to the right. They are:{(-4,14), (-2,8), (0,2),(2,-4),(4,-10)}Range means possible y values, so Range = {-10,-4,2,8,14}

-4

-3

-2

-1

0

1

2

3

4

5

6

7

8

-1 0 1 2 3 4 5 6 7

This figure is a line segment with endpoints(1,-3) and (6,7).

x f(x) = -3x + 2-4 14-2 80 22 -44 -10

-5 -4 -3 -2 -1 0 1 2 3 4 5

-16

-12

-8

-4

0

4

8

12

16

Series1; 14

8

2

-4

-10

f(x) = -3x + 2

x-axis

y-ax

is