Problem 7E

7. The total cost of repaying a student loan at an interest rate of $r \%$ per year is $C=f(r)$.

(a) What is the meaning of the derivative $f^{\prime}(r) ?$ What are its units?

(b) What does the statement $f^{\prime}(10)=1200$ mean?

(c) Is $f^{\prime}(r)$ always positive or does it change sign?

 

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