What if you were given a table of x and y values? How could you write a rule to describe the relationship between the two variables? After completing this Concept, you'll be able to write a function rule for tables like this.
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CK-12 Foundation: 0111S Write a Function Rule
Guidance
In many situations, we collect data by conducting a survey or an experiment, and then organize the data in a table of values. Most often, we want to find the function rule or formula that fits the set of values in the table, so we can use the rule to predict what could happen for values that are not in the table.
Example A
Write a function rule for the following table:
Solution
You pay $24 for 2 CDs, $48 for 4 CDs, $120 for 10 CDs. That means that each CD costs $12.
We can write a function rule:
Cost (number of CDs) or
Example B
Write a function rule for the following table:
Solution
You can see that a negative number turns into the same number, only positive, while a non-negative number stays the same. This means that the function being used here is the absolute value function: .
Coming up with a function based on a set of values really is as tricky as it looks. There’s no rule that will tell you the function every time, so you just have to think of all the types of functions you know and guess which one might be a good fit, and then check if your guess is right. In this book, though, we’ll stick to writing functions for linear relationships, which are the simplest type of function.
Represent a Real-World Situation with a Function
Let’s look at a few real-world situations that can be represented by a function.
Example C
Maya has an internet service that currently has a monthly access fee of $11.95 and a connection fee of $0.50 per hour. Represent her monthly cost as a function of connection time.
Solution
Define
Let the number of hours Maya spends on the internet in one month
Let Maya’s monthly cost
Translate
The cost has two parts: the one-time fee of $11.95 and the per-hour charge of $0.50. So the total cost is the flat fee the charge per hour the number of hours.
Answer
The function is .
Example D
Alfredo wants a deck build around his pool. The dimensions of the pool are and the decking costs $3 per square foot. Write the cost of the deck as a function of the width of the deck.
Solution
Define
Let width of the deck
Let cost of the deck
Make a sketch and label it
Translate
You can look at the decking as being formed by several rectangles and squares. We can find the areas of all the separate pieces and add them together:
To find the total cost, we then multiply the area by the cost per square foot ($3).
Answer
Watch this video for help with the Examples above.
CK-12 Foundation: Write a Function Rule
Vocabulary
- Most often, we want to find the function rule or formula that fits the set of values in the table, so we can use the rule to predict what could happen for values that are not in the table.
Guided Practice
A cell phone company sells two million phones in their first year of business. The number of phones they sell doubles each year. Write a function that gives the number of phones that are sold per year as a function of how old the company is.
Solution
Define
Let age of company in years
Let number of phones that are sold per year
Make a table
Write a function rule
The number of phones sold per year doubles every year, so the first year the company sells 2 million phones, the next year it sells million, the next year it sells million, and so on. You might remember that when we multiply a number by itself several times we can use exponential notation: and so on. In this problem, the exponent just happens to match the company’s age in years, which makes our function easy to describe.
Answer
Practice
- Write a function rule for the following table:
- Write a function rule for the following table:
- Write a function rule for the following table:
- Write a function that represents the number of cuts you need to cut a ribbon into pieces.
- Write a function that represents the number of cuts you need to divide a pizza into slices.
For 6-8, suppose Solomon charges a $40 flat rate plus $25 per hour to repair a leaky pipe.
- Write a function that represents the total fee charged as a function of hours worked.
- How much does Solomon earn for a 3-hour job?
- How much does he earn for three separate 1-hour jobs?
For 9-12, suppose Rochelle has invested $2500 in a jewelry making kit. She makes bracelets that she can sell for $12.50 each.
- Write a function that shows how much money Rochelle makes from selling bracelets.
- Write a function that shows how much money Rochelle has after selling bracelets, minus her investment in the kit.
- How many bracelets does Rochelle need to make before she breaks even?
- If she buys a $50 display case for her bracelets, how many bracelets does she now need to sell to break even?