1. Direct Variation Direct Variation
2.8: Modeling Using Variation
Direct Variation
Certain formulas occur so frequently in applied situations that they are given special names. Variation formulas show how one quantity changes in relation to other quantities. Quantities can vary directly, inversely, or jointly.
Direct Variation
If a situation is described by an equation in the form
y = kx
where k is a constant, we say that y varies directly as x. y is directly proportional to x. The number k is called the constant of variation.

2. Direct Variation Solving Variation Problems
2.8: Modeling Using Variation
Direct Variation
Our work up to this point provides a step-by-step procedure for solving variation problems. This procedure applies to direct variation problems as well as to the other kinds of variation problems that we will discuss.
Solving Variation Problems
1. Write an equation that describes the given English statement.
2. Substitute the given pair of values into the equation in step 1 and find the value of k.
3. Substitute the value of k into the equation in step 1.
4. Use the equation from step 3 to answer the problem's question.

3. Direct Variation Direct Variation with Powers
2.8 : Modeling Using Variation
Direct Variation
Direct variation with powers is modeled by polynomial functions.
Direct Variation with Powers
y varies directly as the nth power of x or y is directly proportional to the nth power of x if there exists some nonzero constant k such that
y = kx n.

4. EXAMPLE: Solving a Direct Variation Problem
2.8 : Modeling Using Variation
EXAMPLE: Solving a Direct Variation Problem
The amount of garbage, G, varies directly as the population, P. Allegheny County, Pennsylvania, has a population of 1.3 million and creates 26 million pounds of garbage each week. Find the weekly garbage produced by New York City with a population of 7.3 million.
Solution
Step Write an equation We know that y varies directly as x is expressed as
y = kx.
By changing letters, we can write an equation that describes the following English statement: Garbage production, G, varies directly as the population, P.
G = kP
more

5. EXAMPLE: Solving a Direct Variation Problem
2.8 : Modeling Using Variation
EXAMPLE: Solving a Direct Variation Problem
The amount of garbage, G, varies directly as the population, P. Allegheny County, Pennsylvania, has a population of 1.3 million and creates 26 million pounds of garbage each week. Find the weekly garbage produced by New York City with a population of 7.3 million.
Solution
Step 2 Use the given values to find k. Allegheny County has a population of 1.3 million and creates 26 million pounds of garbage weekly. Substitute 26 for G and 1.3 for P in the direct variation equation. Then solve for k.
more

6. EXAMPLE: Solving a Direct Variation Problem
2.8 : Modeling Using Variation
EXAMPLE: Solving a Direct Variation Problem
The amount of garbage, G, varies directly as the population, P. Allegheny County, Pennsylvania, has a population of 1.3 million and creates 26 million pounds of garbage each week. Find the weekly garbage produced by New York City with a population of 7.3 million.
Solution
Step 3 Substitute the value of k into the equation.
G = kP Use the equation from step 1.
G = 20P Replace k, the constant of variation, with 20.
more

7. EXAMPLE: Solving a Direct Variation Problem
2.8 : Modeling Using Variation
EXAMPLE: Solving a Direct Variation Problem
The amount of garbage, G, varies directly as the population, P. Allegheny County, Pennsylvania, has a population of 1.3 million and creates 26 million pounds of garbage each week. Find the weekly garbage produced by New York City with a population of 7.3 million.
Solution
Step 4 Answer the problem's question New York City has a population of 7.3 million. To find its weekly garbage production, substitute 7.3 for P in
G = 20P and solve for G.
G = 20P Use the equation from step 3.
G = 20(7.3) Substitute 7.3 for P.
G = 146
The weekly garbage produced by New York City weighs approximately 146 million pounds.

8. Inverse Variation Inverse Variation
2.8 : Modeling Using Variation
Inverse Variation
When two quantities vary inversely, one quantity increases as the other decreases, and vice versa. Generalizing, we obtain the following statement.
Inverse Variation
If a situation is described by an equation in the form
y =
where k is a constant, we say that y varies inversely as x or y is inversely proportional to x. The number k is called the constant of variation.
We use the same procedure to solve inverse variation problems as we did to solve direct variation problems.

9. EXAMPLE: Solving an Inverse Variation Problem
2.8 : Modeling Using Variation
EXAMPLE: Solving an Inverse Variation Problem
To continue making money, the number of new songs, S, a rock band needs to record each year varies inversely as the number of years, N, the band has been recording. After 4 years of recording, a band needs to record 15 new songs per year to be profitable. After 6 years, how many new songs will the band need to record in order to make a profit in the seventh year?
By changing letters, we can write an equation that describes the following English statement: The number of new songs each year, S, varies inversely as the number of years, N.
Step 1 Write an equation We know that y varies inversely as x is expressed as
Solution
more

10. EXAMPLE: Solving an Inverse Variation Problem
2.8 : Modeling Using Variation
EXAMPLE: Solving an Inverse Variation Problem
To continue making money, the number of new songs, S, a rock band needs to record each year varies inversely as the number of years, N, the band has been recording. After 4 years of recording, a band needs to record 15 new songs per year to be profitable. After 6 years, how many new songs will the band need to record in order to make a profit in the seventh year?
Solution
Step 2 Use the given values to find k After 4 years of recording, the band needs to record 15 new songs. Substitute 15 for S and 4 for N in the inverse variation equation. Then solve for k.
more

11. EXAMPLE: Solving an Inverse Variation Problem
2.8 : Modeling Using Variation
EXAMPLE: Solving an Inverse Variation Problem
To continue making money, the number of new songs, S, a rock band needs to record each year varies inversely as the number of years, N, the band has been recording. After 4 years of recording, a band needs to record 15 new songs per year to be profitable. After 6 years, how many new songs will the band need to record in order to make a profit in the seventh year?
Solution
Step 3 Substitute the value of k into the equation.
more

12. EXAMPLE: Solving an Inverse Variation Problem
2.8 : Modeling Using Variation
EXAMPLE: Solving an Inverse Variation Problem
To continue making money, the number of new songs, S, a rock band needs to record each year varies inversely as the number of years, N, the band has been recording. After 4 years of recording, a band needs to record 15 new songs per year to be profitable. After 6 years, how many new songs will the band need to record in order to make a profit in the seventh year?
Solution
Step 4 Answer the problem's question. We need to find how many new songs will the band need to record after 6 years in order to make a profit in the seventh year. Substitute 6 for N in the equation from step 3 and solve for S.
The band will need to record 10 new songs after 6 years.

13. Joint Variation Joint Variation
2.8 : Modeling Using Variation
Joint Variation
Joint Variation
Joint variation is a variation in which a variable varies directly as the product of two or more other variables. Thus, the equation y = kxz is read "y varies jointly as x and z."

14. EXAMPLE: Modeling Centrifugal Force
2.8 : Modeling Using Variation
EXAMPLE: Modeling Centrifugal Force
The centrifugal force, C, of a body moving in a circle varies jointly with the radius of the circular path, r, and the body's mass, m, and inversely with the square of the time, t, it takes to move about one full circle. A 6-gram body moving in a circle with radius 100 centimeters at a rate of 1 revolution in 2 seconds has a centrifugal force of 6000 dynes. Find the centrifugal force of an 18-gram body moving in a circle with radius 100 centimeters at a rate of 1 revolution in 3 seconds.
more

15. EXAMPLE: Modeling Centrifugal Force
2.8 : Modeling Using Variation
EXAMPLE: Modeling Centrifugal Force
Solution
Translate "Centrifugal force, C, varies jointly with radius, r, and mass, m, and inversely with the square of time, t."
Solve for k.
Substitute 40 for k in the model for centrifugal force.
Find C when r = 100, m = 18, and t = 3.
If r = 100, m = 6, and t = 2, then C = 6000.
The centrifugal force is 8000 dynes.

16. Print off this page as proof of you watching the presentation
Print off this page as proof of you watching the presentation. Have it in your hand as soon as you enter the room Name__________________________________