1. Identify, write, and graph an equation of direct variation.
Objective:
Identify, write, and graph an equation of direct variation.

2. Vocabulary
Direct variation – a linear relationship between two variable that can be written in the form y = kx or k = , where k ≠ 0.
Constant of variation – The fixed number k in a direct variation equation.

3. Notes
Reading Math
You can read direct variation as “y varies directly as x” or “y is directly proportional to x” or “y varies with x.”

4. Direct Variation or Not?
Tell whether each equation represents a direct variation. If so, identify the constant of variation.
y + 8 = x
Solve the equation for y. Subtract 8 from both sides.
y + 8 = x
– 8 = – 8
y = x – 8
The equation is not in the form y = kx, so y + 8 = x is not a direct variation.

5. Direct Variation or Not?
Tell whether each equation represents a direct variation. If so, identify the constant of variation.
3y = 2x
Solve the equation for y. Divide both sides by 3.
3y = 2x
y = x
2 3
Write as x .
2x 3
2 3
The equation is in the form y = kx, so the original equation 3y = 2x is a direct variation.

6. What About This One?
Tell whether each equation represents a direct variation. If so, identify the constant of variation.
y + 3 = 3x
y + 3 = 3x
Solve the equation for y. Subtract 3 from both sides.
– 3 – 3
y = 3x – 3
The equation is not in the form y = kx, so y + 3 = 3x is not a direct variation.

7. And, This One?
Tell whether each equation represents a direct variation. If so, identify the constant of variation.
4y = 3x
Solve the equation for y. Divide both sides by 4.
4y = 3x
y = x
3 4
Write as x .
3x 4
3 4
The equation is in the form y = kx, so the original equation 4y = 3x is a direct variation.

8. From a Chart…
Tell whether each set of data represents a direct variation. If so, identify the constant of variation and then write the direct variation equation.
Price (c) 69 99
129
Weight (oz) 2 3 4
Find for each ordered pair.
y x = y x 2 69 = y x 3 99 1 33 = y x 4
129
k is not the same for each ordered pair.
The data does not represent a direct variation.

9. Notes
Helpful Hint
In a direct variation where k is positive, when x increases, y also increases; when x decreases, y also decreases.

10. Now You Try!
Tell whether each set of data represents a direct variation. If so, identify the constant of variation and then write the direct variation equation.
Inches 1 2 5
Centimeters
2.54
5.08
12.70
Find for each ordered pair.
y x y
= = 2.54 x
2.54 1
= = 2.54
5.08 2 y x
= = 2.54
12.7 5 y x
k = 2.54 for each ordered pair.
The data represent a direct variation where k = The equation is y = 2.54x

11. Exit Ticket
Tell whether each set of data represents a direct variation. If so, identify the constant of variation and then write the direct variation equation.
Meters 3 4 5
Miles 9 12 15
Find for each ordered pair.
y x
= = 3 9 3 x y
= = 3 12 4 x y
= = 3 15 5 x y
k = 3 for each ordered pair.
The data represent a direct variation where k = 3. The equation is y = 3x

12. Notes
Helpful Hint
In a direct variation, the slope, k, represents a constant rate of change.

13. Looking at a Graph
Tell whether each graph represents a direct variation. If so, identify the constant of variation and then write the direct variation equation. x y –2 –4 2 4
The graph is a line through (0, 0). This is a direct variation. The Slope of the line is ½, so k = –½ . The equation is y = -½x.

14. Looking at a Graph
Tell whether each graph represents a direct variation. If so, identify the constant of variation and then write the direct variation equation. x y –2 –4 2 4
The line does not pass through (0, 0). This is not a direct variation.

15. Writing an Equation A truck travels at a speed of 55 miles per hour.
Write a direct variation equation for the distance y the truck travels in x hours.
distance =
55 miles per hour
times
number of hours
Use the formula y = kx. k = 55
y = ● x
y = 55x

16. Now Graph It! A truck travels at a speed of 55 miles per hour.
Graph the data.
Make a table. Since time cannot be negative, use nonnegative number for x.

17. Now Graph It!
Use the ordered pairs to plot the points on a coordinate plane. Connect the points in a straight line. Label the axes.
Check
100
y = 55x is in slope-intercept form with m = 55 and b = 0. The graph shows a slope of 55 and a y-intercept of 0. 75
Distance (mi)‏ 50 25 2 4 6 8
Time (h)‏

18. Answer Using Equation
How long does it take the truck to travel 660 miles?
Find the value of x when y = 660
Write the equation for the direct variation.
y = 55x
660 = 55x
Substitute 660 for y.
Divide both sides by 660.
12 = x
It will take the truck 12 hours to travel 660 miles.

19. Exit Ticket A bicycle travels at a speed of 12 miles per hour.
1) Write a direct variation equation for the distance y the bike travels in x hours.
distance =
12 miles per hour
times
number of hours
Use the formula y = kx. k = 12
2) Make a table and graph the data.