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Ben has a cell phone plan that provides 200 free minutes each month for a flat rate of [tex] \[tex]$39 [/tex]. For any minutes over 200, Ben is charged [tex] \$[/tex]0.35 [/tex] per minute. Which of the following piecewise functions accurately represents charges based on Ben's cell phone plan?

A. [tex] f(x)=\left\{\begin{array}{ll} 39, & x \leq 200 \\ 39 + 0.35(x-200), & x \ \textgreater \ 200 \end{array}\right\} [/tex]

B. [tex] f(x)=\left\{\begin{array}{ll} 39, & x \ \textgreater \ 200 \\ 39 + 0.35, & x \leq 200 \end{array}\right\} [/tex]

C. [tex] f(x)=\left\{\begin{array}{ll} 39, & x \leq 200 \\ 0.35(x-200), & x \ \textgreater \ 200 \end{array}\right\} [/tex]

D. [tex] f(x)=\left\{\begin{array}{ll} 39, & x \leq 200 \\ 0.35x, & x \ \textgreater \ 200 \end{array}\right\} [/tex]

Sagot :

To determine which piecewise function accurately represents Ben's cell phone plan charges, let's break down the conditions and charges described.

1. Ben pays a flat rate of \$39 for up to 200 minutes each month.
2. For any minutes over 200, he is charged an additional \$0.35 per minute.

We need to construct the piecewise function \( f(x) \) that encompasses this information, where \( x \) represents the total number of minutes used in a month.

Step-by-Step Solution:

- If \( x \leq 200 \):

Ben uses 200 minutes or less, so the cost is simply the flat rate of \$39. Thus:
[tex]\[ f(x) = 39 \quad \text{for } x \leq 200 \][/tex]

- If \( x > 200 \):

Ben uses more than 200 minutes, so in addition to the flat rate of \[tex]$39, he has to pay an extra \$[/tex]0.35 for each minute over 200. For instance, if he uses \( x \) minutes, the number of minutes over 200 is \( x - 200 \). Therefore, the additional charge is:
[tex]\[ \text{Additional charge} = 0.35 \times (x - 200) \][/tex]
Hence, the total cost in this case is:
[tex]\[ f(x) = 39 + 0.35 \times (x - 200) \quad \text{for } x > 200 \][/tex]

Putting this together, the piecewise function should be written as:
[tex]\[ f(x) = \begin{cases} 39, & \text{if } x \leq 200 \\ 39 + 0.35(x - 200), & \text{if } x > 200 \end{cases} \][/tex]

Comparing this with the given options:

- Option A matches this piecewise function exactly:
[tex]\[ f(x) = \begin{cases} 39, & x \leq 200 \\ 39 + 0.35(x - 200), & x > 200 \end{cases} \][/tex]

- Option B is incorrect because it incorrectly places the conditions and does not make sense logically according to the problem's description.

- Option C is incorrect because it only accounts for the additional per-minute charge without adding the flat rate.

- Option D is incorrect because it misrepresents the additional charges beyond 200 minutes.

Thus, the correct answer is:
- A. \( f(x) = \begin{cases}
39, & x \leq 200 \\
39 + 0.35(x - 200), & x > 200
\end{cases} \)
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