ultimate guide to coordinate plane

Review

The Coordinate Plane

Everything you want need to know about writing, graphing, & solving equations of lines.

Includes Bonus Content:Your ultimate guide to

Parallel & Perpendicular Lines

EXAMPLE 1 Use slope and y-intercept to write an equation

Write an equation of the line with a slope of –2 and a y-intercept of 5.

y = mx + b Write slope-intercept form.

y = –2x + 5 Substitute –2 for m and 5 for b.

EXAMPLE 2 Standardized Test Practice

Which equation represents the line shown?

The line crosses the y-axis at (0, 3). So, the y-intercept is 3.

y = mx + b Write slope-intercept form.

2y = – x + 35

2Substitute – for m and 3 for b.5

= = –The slope of the line is riserun

–2

5

25

.

A y = – x + 325

B y = – x + 352

C y = – x + 125

D y = 3x + 25

EXAMPLE 2 Standardized Test Practice

ANSWER

The correct answer is A. B DCA

GUIDED PRACTICE for Examples 1 and 2

1. Slope is 8; y-intercept is –7.

Write an equation of the line with the given slope and y-intercept.

y = 8x – 7 ANSWER

GUIDED PRACTICE for Examples 1 and 2

2. Slope is ; y intercept is –3. 34

Write an equation of the line with the given slope and y-intercept.

y = 34

x – 3ANSWER

EXAMPLE 3 Write an equation of a line given two points

Write an equation of the line shown.

Write an equation of a line given two points

y = mx + b

y = x – 543 Substitute for m and 5 for b.4

3

STEP 1

Write an equation of the line. The line crosses the y-axis at (0, –5). So, the y-intercept is –5.

SOLUTION

x2 – x1 33 – 0

y2 – y1=m =

–1 – (–5)=

4

Write slope-intercept form.

EXAMPLE 3

Calculate the slope.

STEP 2

SOLUTION

Write a linear function

Write an equation for the linear function f with the values f(0) = 5 and f(4) = 17.

Calculate the slope of the line that passes through (0, 5) and (4, 17).

Write f(0) = 5 as (0, 5) and f (4) = 17 as (4, 17).

EXAMPLE 4

STEP 1

x2 – x1 4 – 0

y2 – y1=m =

17 – 5=

4

12= 3

STEP 2

y = mx + b Write slope-intercept form.

y = 3x + 5 Substitute 3 for m and 5 for b.

Write an equation of the line. The line crosses the y-axis at (0, 5). So, the y-intercept is 5.

STEP 3

EXAMPLE 4 Write a linear function

ANSWER

The function is f(x) = 3x + 5.

GUIDED PRACTICE for Examples 3 and 4

3. Write an equation of the line shown.

2

1 x + 1y =ANSWER –

GUIDED PRACTICE for Examples 3 and 4

Write an equation for the linear function f with the given values.4.

f(0) = –2, f(8) = 4

y = x – 234

ANSWER

GUIDED PRACTICE for Examples 3 and 4

Write an equation for the linear function f with the given values.5.

f(–3) = 6, f(0) = 5

y = x + 5 13

ANSWER –

EXAMPLE 1 Write an equation given the slope and a point

Write an equation of the line that passes through the point (–1, 3) and has a slope of –4.

SOLUTION

y = mx + b Write slope-intercept form.

Substitute –4 for m, –1 for x, and 3 for y.

3 = –4(–1) + b

Identify the slope. The slope is – 4.STEP 1

Find the y-intercept. Substitute the slope and the coordinates of the given point in y = mx + b. Solve for b.

STEP 2

EXAMPLE 1

–1 = b Solve for b.

y = mx + b Write slope-intercept form.

Substitute –4 for m and –1 for b.y = –4x – 1

Write an equation of the line.STEP 3

Write an equation given the slope and a point

GUIDED PRACTICE for Example 1

Write an equation of the line that passes through the point (6, 3) and has a slope of 2.

y = 2x – 9ANSWER

EXAMPLE 2 Write an equation given two points

Write an equation of the line that passes through (–2, 5) and (2, –1).

SOLUTION

Calculate the slope.

3m =

y2 – y1

x2 – x1

= –1 – 52 – (–2)

= –64

= –2

Find the y-intercept. Use the slope and the point (–2, 5).

y = mx + b Write slope-intercept form.

STEP 1

STEP 2

EXAMPLE 2

5 = –32

(–2) + b

2 = b Solve for b.

Write an equation of the line.

y = mx + b Write slope-intercept form.

Substitute – 32

for m and 2 for b.y = – 32

x + 2

STEP 3

Substitute – for m, –2 for x, and 5 for y.

32

Write an equation given two points

EXAMPLE 3

Which function has the values f(4) = 9 and f(–4) = –7?

C f (x) = 2x – 13

m =y2 – y1

x2 – x1

= –7 – 9–4 – 4

= –16–8

= 2

D f (x) = 2x – 14

B f (x) = 2x + 1

Find the y-intercept. Use the slope and the point (4, 9).

y = mx + b Write slope-intercept form.

EXAMPLE 3 Standardized Test Practice

f (x) = 2x + 10A

STEP 1 Calculate the slope. Write f (4) = 9 as (4, 9) and f (–4) = –7 as (–4, –7).

STEP 2

Substitute 2 for m, 4 for x, and 9 for y.9 = 2(4) + b

1 = b Solve for b.

Write an equation for the function. Use function notation.

f (x) = 2x + 1 Substitute 2 for m and 1 for b.

ANSWER

The answer is B. A C DB

EXAMPLE 3 Standardized Test Practice

STEP 3

GUIDED PRACTICE for Examples 2 and 3

2. Write an equation of the line that passes through (1, –2) and (–5, 4).

y = –x – 1ANSWER

GUIDED PRACTICE for Examples 2 and 3

3. Write an equation for the linear function with values f(–2) = 10 and f(4) = –2?

y = –2x + 6ANSWER

Graph an equation in point-slope formEXAMPLE 2

y + 2 = (x – 3).2 3

Graph the equation

SOLUTION

Because the equation is in point-slope form, you know that the line has a slope of and passes through the point (3, –2).

2 3

Plot the point (3, –2). Find a secondpoint on the line using the slope.Draw a line through both points.

Graph an equation in point-slope form

EXAMPLE 2

y – 1 = (x – 2).–Graph the equation2.

GUIDED PRACTICE for Example 2

ANSWER

SOLUTION

EXAMPLE 1 Write an equation of a parallel line

Write an equation of the line that passes through (–3, –5) and is parallel to the line y = 3x – 1.

STEP 1

Identify the slope. The graph of the given equation has a slope of 3. So, the parallel line through (–3, –5) has a slope of 3.

STEP 2

Find the y-intercept. Use the slope and the given point.

EXAMPLE 1 Write an equation of a parallel line

y = mx + b

–5 = 3(–3) + b

4 = b

Write slope-intercept form.

Substitute 3 for m, 3 for x, and 5 for y.

Solve for b.

STEP 3

Write an equation. Use y = mx + b.

y = 3x + 4 Substitute 3 for m and 4 for b.

GUIDED PRACTICE for Example 1

1. Write an equation of the line that passes through

(–2, 11) and is parallel to the line y = –x + 5.

y = –x + 9ANSWER

SOLUTION

EXAMPLE 3 Determine whether lines are perpendicular

Line a: 12y = –7x + 42

Line b: 11y = 16x – 52

Find the slopes of the lines. Write the equations in slope-intercept form.

The Arizona state flag is shown in a coordinate plane. Lines a and b appear to be perpendicular. Are they?

STATE FLAG

EXAMPLE 3 Determine whether lines are perpendicular

Line a: 12y = –7x + 42

Line b: 11y = 16x – 52

y = – x +12

42 7

12

11

52y = x –

16

11

ANSWER

The slope of line a is – . The slope of line b is

The two slopes are not negative reciprocals, so lines a and b are not perpendicular.

712

1611

SOLUTION

EXAMPLE 4 Write an equation of a perpendicular line

Write an equation of the line that passes through (4, –5) and is perpendicular to the line y = 2x + 3.

STEP 1

Identify the slope. The graph of the given equation has a slope of 2. Because the slopes of perpendicular lines are negative reciprocals, the slope of the perpendicular line through (4, –5) is .

12

–

EXAMPLE 4

STEP 2 Find the y-intercept. Use the slope and thegiven point.

Write slope-intercept form.

–5 = – (4) + b12 Substitute – for m, 4 for x, and

–5 for y.

12

y = mx + b

–3 = b Solve for b.

STEP 3 Write an equation.

y = mx + b Write slope-intercept form.

y = – x – 312 Substitute – for m and –3 for b.1

2

Write an equation of a perpendicular line

GUIDED PRACTICE for Examples 3 and 4

3. Is line a perpendicular to line b? Justify your answer using slopes.

Line a: 2y + x = –12

Line b: 2y = 3x – 8

ANSWER

No; the slope of line a is – , the slope of line b is . The slopes are not negative reciprocals so the lines are not perpendicular.

12

32

GUIDED PRACTICE for Examples 3 and 4

4. Write an equation of the line that passes through (4, 3) and is perpendicular to the line y = 4x – 7.

y = – x + 414

ANSWER

EXAMPLE 2 Determine whether lines are parallel or perpendicular

Determine which lines, if any, are parallel or perpendicular.

Line a: y = 5x – 3

Line b: x + 5y = 2

Line c: –10y – 2x = 0

SOLUTION

Find the slopes of the lines.

Line a: The equation is in slope-intercept form. The slope is 5.

Write the equations for lines b and c in slope-intercept form.

EXAMPLE 2

Line b: x + 5y = 2

5y = – x + 2

Line c: –10y – 2x = 0

–10y = 2x

y = – x15

Determine whether lines are parallel or perpendicular

xy =25

15

+–

EXAMPLE 2

ANSWER

Lines b and c have slopes of – , so they are

parallel. Line a has a slope of 5, the negative reciprocal

of – , so it is perpendicular to lines b and c.

15

15

Determine whether lines are parallel or perpendicular

GUIDED PRACTICE for Example 2

Determine which lines, if any, are parallel or perpendicular.

Line a: 2x + 6y = –3

Line b: y = 3x – 8

Line c: –1.5y + 4.5x = 6

ANSWER

parallel: b and c; perpendicular: a and b, a and c

LIBRARY

EXAMPLE 5

Your class is taking a trip to the public library. You can travel in small and large vans. A small van holds 8 people and a large van holds 12 people. Your class could fill 15 small vans and 2 large vans.

b. Graph the equation from part (a).

c. List several possible combinations.

Solve a multi-step problem

Write an equation in standard form that models the possible combinations of small vans and large vans that your class could fill.

a.

SOLUTION

a. Write a verbal model. Then write an equation.

Because your class could fill 15 small vans and 2 large vans, use (15, 2) as the s- and l-values to substitute in the equation 8s + 12l = p to find the value of p.

8(15) + 12(2) = p Substitute 15 for s and 2 for l.144 = p Simplify.

Substitute 144 for p in the equation 8s + 12l = p.

EXAMPLE 5 Solve a multi-step problem

8 s l p12+ =

Substitute 0 for s.

8(0) + 12l = 144

l = 12

Substitute 0 for l.

s = 188s + 12(0) = 144

ANSWER

The equation 8s + 12l = 144 models the possible combinations.

b. Find the intercepts of the graph.

EXAMPLE 5 Solve a multi-step problem

EXAMPLE 5 Solve a multi-step problem

Plot the points (0, 12) and (18, 0). Connect them with a line segment. For this problem only nonnegative whole-number values of s and l make sense.

The graph passes through (0, 12), (6, 8), (12, 4), and (18, 0). So, four possible combinations are 0 small and 12 large, 6 small and 8 large, 12 small and 4 large, 18 small and 0 large.

c.

EXAMPLE 5 Solve a multi-step problemEXAMPLE 5 Solve a multi-step problemGUIDED PRACTICE for Example 5

7. WHAT IF? In Example 5, suppose that 8 students decide not to go on the class trip. Write an equation that models the possible combinations of small and large vans that your class could fill. List several possible combinations.

8s + 12l = 136; 17 small, 0 large; 14 small, 2 large; 11 small, 4 large; 8 small, 6 large; 5 small, 8 large; 2 small, 10 large

ANSWER