1. 12-1 Inverse Variation Warm Up Lesson Presentation Lesson Quiz
Holt Algebra 1

2. Warm Up Solve each proportion. 1. 2. 4.2 10 3. 2.625 4. 2.5
5. The value of y varies directly with x, and y = –6 when x = 3. Find y when x = –4. 8
6. The value of y varies directly with x, and y = 6 when x = 30. Find y when x = 45. 9

3. Objective
Identify, write, and graph inverse operations.

4. Vocabulary
inverse variation

5. A relationship that can be written in the form y = , where k is a nonzero constant and x ≠ 0, is an inverse variation. The constant k is the constant of variation. Inverse variation implies that one quantity will increase while the other quantity will decrease (the inverse, or opposite, of increase).
Multiplying both sides of y = by x gives xy = k. So, for any inverse variation, the product of x and y is a nonzero constant.

6. A direct variation is an equation that can be written in the form y = kx, where k is a nonzero constant.
Remember!

7. There are two methods to determine whether a relationship between data is an inverse variation. You can write a function rule in y = form, or you can check whether xy is a constant for each ordered pair.

8. Example 1A: Identifying an Inverse Variation
Tell whether each relationship is an inverse variation. Explain.
Method 1 Write a function rule.
Can write in y = form.
The relationship is an inverse variation.
Method 2 Find xy for each ordered pair.
1(30) = 30, 2(15) = 30, 3(10) = 30
The product xy is constant, so the relationship is an inverse variation.

9. Example 1B: Identifying an Inverse Variation
Tell whether each relationship is an inverse variation. Explain.
Method 1 Write a function rule.
Cannot write in y = form.
y = 5x
The relationship is not an inverse variation.
Method 2 Find xy for each ordered pair.
1(5) = 5, 2(10) = 20, 4(20) = 80
The product xy is not constant, so the relationship is not an inverse variation.

10. Example 1C: Identifying an Inverse Variation
Tell whether each relationship is an inverse variation. Explain.
2xy = 28
Find xy. Since xy is multiplied by 2, divide both sides by 2 to undo the multiplication.
xy = 14
Simplify.
xy equals the constant 14, so the relationship is an inverse variation.

11. Check It Out! Example 1a
Tell whether each relationship is an inverse variation. Explain.
Method 1 Write a function rule.
Cannot write in y = form.
y = –2x
The relationship is not an inverse variation.
Method 2 Find xy for each ordered pair.
–12 (24) = –228 , 1(–2) = –2, 8(–16) = –128
The product xy is not constant, so the relationship is not an inverse variation.

12. Check It Out! Example 1b
Tell whether each relationship is an inverse variation. Explain.
Method 1 Write a function rule.
Can write in y = form.
The relationship is an inverse variation.
Method 2 Find xy for each ordered pair.
3(3) = 9, 9(1) = 9, 18(0.5) = 9
The product xy is constant, so the relationship is an inverse variation.

13. Check It Out! Example 1c
Tell whether each relationship is an inverse variation. Explain.
Cannot write in y = form.
2x + y = 10
The relationship is not an inverse variation.

14. Since k is a nonzero constant, xy ≠ 0
Since k is a nonzero constant, xy ≠ 0. Therefore, neither x nor y can equal 0, and no solution points will be on the x- or y-axes.
Helpful Hint

15. An inverse variation can also be identified by its graph
An inverse variation can also be identified by its graph. Some inverse variation graphs are shown. Notice that each graph has two parts that are not connected.
Also notice that none of the graphs contain (0, 0). This is because (0, 0) can never be a solution of an inverse variation equation.

16. Example 2: Graphing an Inverse Variation
Write and graph the inverse variation in which y = 0.5 when x = –12.
Step 1 Find k.
k = xy
Write the rule for constant of variation.
= –12(0.5)
Substitute –12 for x and 0.5 for y.
= –6
Step 2 Use the value of k to write an inverse variation equation.
Write the rule for inverse variation.
Substitute –6 for k.

17. Write and graph the inverse variation in which y = 0.5 when x = –12.
Example 2 Continued
Write and graph the inverse variation in which y = 0.5 when x = –12.
Step 3 Use the equation to make a table of values. y –2 –4 x –1 1 2 4
1.5 3 6
undef. –6 –3
–1.5

18. Write and graph the inverse variation in which y = 0.5 when x = –12.
Example 2 Continued
Write and graph the inverse variation in which y = 0.5 when x = –12.
Step 4 Plot the points and connect them with smooth curves. ●

19. Check It Out! Example 2
Write and graph the inverse variation in which y = when x = 10.
Step 1 Find k.
k = xy
Write the rule for constant of variation.
= 10
Substitute 10 for x and for y.
= 5
Step 2 Use the value of k to write an inverse variation equation.
Write the rule for inverse variation.
Substitute 5 for k.

20. Check It Out! Example 2 Continued
Write and graph the inverse variation in which y = when x = 10.
Step 3 Use the equation to make a table of values. x –4 –2 –1 1 2 4 y
–1.25
–2.5 –5
undef. 5
2.5
1.25

21. Check It Out! Example 2 Continued
Write and graph the inverse variation in which y = when x = 10.
Step 4 Plot the points and connect them with smooth curves. ●

22. Example 3: Transportation Application
The inverse variation xy = 350 relates the constant speed x in mi/h to the time y in hours that it takes to travel 350 miles. Determine a reasonable domain and range and then graph this inverse variation. Use the graph to estimate how long it will take to travel 350 miles driving 55 mi/h.
Step 1 Solve the function for y so you can graph it.
xy = 350
Divide both sides by x.

23. Example 3 Continued
Step 2 Decide on a reasonable domain and range.
x > 0
Length is never negative and x ≠ 0
y > 0
Because x and xy are both positive, y is also positive.
Step 3 Use values of the domain to generate reasonable ordered pairs.
4.38
5.83
8.75
17.5 y 80 60 40 20 x

24. Step 4 Plot the points. Connect them with a smooth curve.
Example 3 Continued
Step 4 Plot the points. Connect them with a smooth curve. ●
Step 5 Find the y-value where x = 55. When the speed is 55 mi/h, the travel time is about 6 hours.

25. Recall that sometimes domain and range are restricted in real-world situations.
Remember!

26. Check It Out! Example 3
The inverse variation xy = 100 represents the relationship between the pressure x in atmospheres (atm) and the volume y in mm³ of a certain gas. Determine a reasonable domain and range and then graph this inverse variation. Use the graph to estimate the volume of the gas when the pressure is 40 atmospheric units.
Step 1 Solve the function for y so you can graph it.
xy = 100
Divide both sides by x.

27. Check It Out! Example 3 Continued
Step 2 Decide on a reasonable domain and range.
x > 0
Pressure is never negative and x ≠ 0
y > 0
Because x and xy are both positive, y is also positive.
Step 3 Use values of the domain to generate reasonable pairs.
2.5
3.34 5 10 y 40 30 20 x

28. Check It Out! Example 3 Continued
Step 4 Plot the points. Connect them with a smooth curve. ●
Step 5 Find the y-value where x = 40. When the pressure is 40 atm, the volume of gas is about 2.5 mm3.

29. The fact that xy = k is the same for every ordered pair in any inverse variation can help you find missing values in the relationship.

31. Example 4: Using the Product Rule
Let and
Let y vary inversely
as x. Find
Write the Product Rule for Inverse Variation.
Substitute 5 for for and 10 for
Simplify.
Solve for by dividing both sides by 5.
Simplify.

32. Check It Out! Example 4
Let and Let y vary inversely as x. Find
Write the Product Rule for Inverse Variation.
Substitute 2 for –4 for and –6 for
Simplify.
Solve for by dividing both sides by –4.
Simplify.

33. Example 5: Physics Application
Boyle’s law states that the pressure of a quantity of gas x varies inversely as the volume of the gas y. The volume of gas inside a container is 400 in3 and the pressure is 25 psi. What is the pressure when the volume is compressed to 125 in3?
Use the Product Rule for Inverse Variation.
(400)(25) = (125)y2
Substitute 400 for for and 25 for
Simplify.
Solve for by dividing both sides by 125.

34. Example 5 Continued
Boyle’s law states that the pressure of a quantity of gas x varies inversely as the volume of the gas y. The volume of gas inside a container is 400 in3 and the pressure is 25 psi. What is the pressure when the volume is compressed to 125 in3?
When the gas is compressed to 125 in3, the pressure increases to 80 psi.

35. Check It Out! Example 5
On a balanced lever, weight varies inversely as the distance from the fulcrum to the weight. The diagram shows a balanced lever. How much does the child weigh?

36. Check It Out! Example 5 Continued
Use the Product Rule for Inverse Variation.
Substitute 3.2 for , 60 for and 4.3 for
Simplify.
Solve for by dividing both sides by 3.2.
Simplify.
The child weighs lb.

37. Lesson Quiz: Part I
1. Write and graph the inverse variation in which y = 0.25 when x = 12.

38. Lesson Quiz: Part II
2. The inverse variation xy = 210 relates the length y in cm to the width x in cm of a rectangle with an area of 210 cm2. Determine a reasonable domain and range and then graph this inverse variation. Use the graph to estimate the length when the width is 14 cm.

39. Lesson Quiz: Part III
3. Let x1= 12, y1 = –4, and y2 = 6, and let y vary inversely as x. Find x2. –8