Coordinate Geometry and Functions

The principal goal of education is to create individuals who are capable of doing new things, not simply of repeating what other generations have done.

Jean Piaget

Swiss psychologist

Introduction to the Cartesian Coordinate (x,y) Plane

Before the end of the European Renaissance, math was cleanly divided into the two separate subjects of geometry and algebra. You didn't use algebraic equations in geometry, and you didn't draw any pictures in algebra.

Then, around 1637, a French guy named René Descartes came up with a way to put these two subjects together

Seating ChartThe following seating chart has 5 rows of desks with 4 desks in each row

Mary sits in the third row at the second desk

Row 1 Row 2 Row 3 Row 4 Row 5

Who is sitting in desk (4,2)?

A B C D E

F G H I J

K

P

L M N O

Q R S T

4

3

2

11 2 3 4 5

N

Row 4

Seat 2

Mary sits in the third row at the second desk (3,2) and George sits in the second row at the third desk (2,3). Are these seats the same?

No!! The seats (3,2) and (2,3) are called ordered pairs because the order in which the pair of numbers is written is important!!

This was a handy way to reference individuals in the room, telling you how far over (row 2) and how far up (seat 4) you needed to go. Descartes did something similar for mathematics

Coordinate Plane

Descartes' breakthrough was in taking a second number line, standing it up on its end, and crossing the first number line at zero:

Coordinate Plane

Definitionx axis – a horizontal number line on a

coordinate grid.

1 2 3 4 50 6x

Definitiony axis – a vertical number line on a

coordinate grid.

12345

0

6

y

Coordinates – an ordered pair of numbers that give the location of a point on a grid. (3, 4)

12345

0

6

1 2 3 4 50 6

(3,4)

Hint The first number is always the x or first letter in

the alphabet. The second number is always the y the second letter in the alphabet.

1

32

45

0

6

1 2 3 4 50 6

(3,4)

How to Plot Ordered PairsStep 1 – Always find the x value first, moving horizontally either right (positive) or left (negative).

1

32

45

0

6

1 2 3 4 50 6

(2, 3)

y

x

How to Plot Ordered PairsStep 2 – Starting from your new position find the y value by moving vertically either up (positive) or down (negative).

1

32

45

0

6

1 2 3 4 50 6

(2, 3)

(2,3)y

x

What is the ordered pair?

1

32

45

0

6

1 2 3 4 50 6

(3,5)

y

x

The x-axis and y-axis separate the coordinate plane into four regions, called quadrants.

II(-, +)

I(+, +)

IV(+, -)

III(-, -)

How to Plot in 4 QuadrantsStep 1 - Plot the x number first moving to the left when the number is negative.

-2

0-1

12

-3

3

-2 -1 0 1 2-3 3

(-3, -2)(-3, -2)y

x

How to Plot in 4 QuadrantsStep 2 - Plot the y number moving from your new position down 2 when the number is negative.

-2

0-1

12

-3

3

-2 -1 0 1 2-3 3

(-3, -2)(-3, -2)y

x

Write the ordered pairs that name points A, B, C, and D.

A = (1, 3)B = (3, -2)C = (0, -4)D = (-6, -1)

5

5

-5

-5

• A

• B• C

• D

Linear Equation: an equation whose graph forms a line in an x,y coordinate plane

x + y = 1

3x – 4y = 2

In linear equations, all variables are taken to the first power.

Suppose you were asked to graph the solution set of the linear equation,

You could start by creating a table of values.

x + y = 3

Choose any number to use as x or y

x + y = 3

x

0

3

-3

y

3

0

6

Substitute into equation and solve

0 + y = 3

y = 3

3 + y = 3

y = 0

-3 + y = 3

y = 6

Plot the points and graph

x

0

3

-3

y

3

0

6

x + y = 3

(0,3)

(3,0)

(-3,6)

connect the points with a straight line

Let’s graph another line:

3x – y = 2

x

-1

0

2

y

-5

-2

4

Substitute into equation and solve

3(-1) – y = 2

y = -5

3(0) – y = 2

y = -2

3(2) – y = 2

y = 4

Plot the points and graph

x

-1

0

2

y

-5

-2

4

3x – y = 2

(0,-2)

(2,4)

(-1,-5)

connect the points with a straight line

Graphing Using InterceptsThe x-intercept of a line is the x-coordinate of the point where the line intercepts the x-axis.

-8 -6 -4 -2

2

42 6 8

4

6

-4

-6

-8

-2

8The line shown intercepts the x-axis at (2,0).

We say that the x-intercept is 2.

Graphing Using InterceptsThe y-intercept of a line is the y-coordinate of the point where the line intercepts the y-axis.

-8 -6 -4 -2

2

42 6 8

4

6

-4

-6

-8

-2

8The line shown intercepts the y-axis at (0,-6).

We say that the y-intercept is -6.

Graph using intercepts.

x y-40

4x – 3y = 12

*To find the y-intercept, let x = 0.

4(0) – 3y = 12

0 – 3y = 12

-3y = 12-3 -3

y = -4

4x – 3y = 12

x y-4

0

0

3

4x – 3y = 12

*To find the x-intercept, let y = 0.

4x – 3(0) = 12

4x - 0 = 12

4x = 124 4

x = 3

Graph using intercepts. 4x – 3y = 12

-8 -6 -4 -2

2

42 6 8

4

6

-4

-6

-8

-2

8

x y-4

0

0

3

Graph using intercepts. 4x – 3y = 12

Slope

Graphs of any line can easily be classified by its y-intercept and by its steepness.

The steepness is often referred to as slope

Types of Slope

PositiveNegative

Zero

Undefinedor

No Slope

Slope

To walk up the steps, the man must go up one step and over one step

1

1

1

1

To walk up the steps, the man must go up two steps and over one step

To walk up the steps, the man must go up one step and over two steps

-4 -2

2

42 6 8 10

4

6

-4

-6

-8

-2

-10

10

8

rise

runSlope =riserun

Slope

Graph the line containing points (2,1) and (7,6) and find the slope.

-4 -2

2

42 6 8 10

4

6

-4

-6

-8

-2

-10

10

8

rise

runSlope =riserun

55

1

Find the slope

Slope =riserun

change in y coordinateschange in x coordinates

2 1

2 1

y ym

x x

1 2

1 2

y ym

x x

or

Slope

Find the slope of the line containing points (1,6) and (5,4).

2 1

2 1

y ym

x x

4 6m

5 1

2m

4

1m

2

Slope of a Horizontal Line

-4 -2

2

42 6 8 10

4

6

-4

-6

-8

-2

-10

10

8What is the rise?

What is the run?

0

6

Slope =riserun

06

0

The slope of ANY horizontal line is 0.

Slope of a Vertical Line

-4 -2

2

42 6 8 10

4

6

-4

-6

-8

-2

-10

10

8What is the rise?

What is the run?

5

0

Slope =riserun

50

The slope of ANY vertical line is undefined.

If given an equation of a line, there is a nice way to find the slope and y-intercept. Simply write the equation in slope-intercept form, which is:

y = mx + b slope

y-intercept

Find the slope andy-intercept of the

following equations.

y = 3x + ½

slope= 3

y-intercept = ½

3x + 5y = 10

First, solve the equation for y.

5y = -3x + 10

y = -3/5 x + 2

m= -3/5 b = 2

Find the slope and y-intercept:

To find the equation, consider slope intercept form: y = mx + b

• Slope• If given, just substitute it into the

equation• Otherwise calculate the slope

• y-intercept• If given, just substitute it into the

equation• Otherwise use a point and

substitute into equation for x and for y, then solve for b

Find the equation of a line:

Always find slope first!

Find the slope using the formula.m = 2 – 6 4 – 3 m = -4

Find the equation of a line containing the following points:

(4,2) and (3,6)

y = -4x + b

m = ?b = ?

(4, 2) and (3, 6) m = -4

Find the y-intercept.y = mx + b2 = -4 H 4 + b2 = -16 + b18 = b

Pick a point to plug into the equation

(4,2) → x = 4y = 2

Write equation:y = -4x + 18

Relations A relation is a mapping, or pairing, of

input values with output values.

The set of input values is called the domain (x values)

The set of output values is called the range (y-values)

Functions

A relation is a function provided there is exactly one output for each input.

It is NOT a function if at least one input has more than one output

INPUT

(DOMAIN)

OUTPUT (RANGE)

FUNCTIONMACHINE

In order for a relationship to be a function…

EVERY INPUT MUST HAVE AN OUTPUT

TWO DIFFERENT INPUTS CAN HAVE THE SAME OUTPUT

ONE INPUT MUST HAVE ONLY ONE OUTPUT

No two ordered pairs can have the same first coordinate

(and different second coordinates).

Which of the following relations are functions?

R= {(9,10, (-5, -2), (2, -1), (3, -9)}

S= {(6, a), (8, f), (6, b), (-2, p)}

T= {(z, 7), (y, -5), (r, 7) (z, 0), (k, 0)}

Identify the Domain and Range. Then tell if the relation is a function.

Input Output

-3 3

1 1

3 -2

4

Domain = {-3, 1,3,4}Range = {3,1,-2}

Function?Yes: each input is mapped onto exactly one output

Input Output

-3 3

1 -2

4 1

4

Identify the Domain and Range. Then tell if the relation is a function.

Domain = {-3, 1,4}Range = {3,-2,1,4}

Function?No: input 1 is mapped onto both -2 & 1

The Vertical Line Test

If it is possible for a vertical line to intersect a graph at more than one point, then the graph is NOT the graph of a function.

Page 117

Which are functions?

Function FunctionNot a Function

“f of x”

Input = x

Output = f(x) = y

Function Notationf (x)

y = 6 – 3x

-2

-1

0

1

2

12

9

6

0

3

x y

f(x) = 6 – 3x

-2

-1

0

1

2

12

9

6

0

3

x f(x)

Before… Now…

(x, y) (x, f(x))

f(x) = 2x2 – 3

Find f(0), f(3)

Evaluate the function:

f(0) = 2(0)2 – 3

= 0 – 3

= - 3

f(3) = 2(3)2 – 3

= 18 – 3

= 15