# cellphoneEqn

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Exploring Linear Equations
A cell phone company charges a flat rate of \$25 a month plus 10 cents a minute.

The linear equation for this charge is:
y  25  .10 x
Normally, the term with the variable in it goes first in the equation, so this would be written as:
y  .10 x  25

Enter this equation in your graphing calculator using the [Y=] button.

1. [Graph] the function.
What does it look like?

2. [2nd], [Graph] to activate the table. Fill in the y-values below.

x         y
-2
-1
0
1
2
3. What real-life measurement does “x” represent in the equation?

4. What real-life measurement does “y” represent in the equation?

5. Draw the line on the graph below.

6. Where does the line cross the x-axis?

Draw a point there and label it (x,y)

7. Where does the line cross the y-axis?
Draw a point there and label it (x,y)

Lynda Aguirre 2009                                                        Calculator Concepts
Name:
8. Push the [Y=] button and enter this equation into Y2=

y  .05x  35

Push [Graph] to see Y1 and Y2 on the same screen.
Look in the table to compare Y1, Y2 values (they’ll be side by side)

9. How does this compare to the first equation?

10. What does this new equation tell you, in real-life terms?

11. If you used 245 minutes last month, how much would each cell phone plan cost you?

12. Which plan is the better bargain?

13. If a third plan charged no flat fees, but cost 8 cents a minute, what would this equation look
like?

14. What is the cost for this third plan if you used 245 minutes last month?

15. Which of the three plans is a better bargain?

16. How would the equation change if the plan charged \$40 for the first 200 minutes and 15
cents for every minute after that? Take a guess.

17. How much would it cost if you used fewer than 200 minutes?

18. How much would it cost if you used 245 minutes?

19. Which is the best plan now?

20. Write the equation for your current phone plan charges.

21. Compare it to other student’s by posting your equation on the PALS message board, who
has the better plan?

Lynda Aguirre 2009                                                    Calculator Concepts

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