In Exercise, consider the following. In the early 1930s, psychologist L. L. Thurstone dete...

In Exercise, consider the following. In the early 1930s, psychologist L. L. Thurstone determined that the time needed to memorize a list of random words is given by the model

where x represents the number of words on the list and f(x) represents the time needed to memorize the list, measured in minutes. The constants a and b are different for each subject and are determined by pretesting.

Psychology—Memorization Rates: Erica is a freshman subject in a psychology class and learns the items on the memorization list according to the model

(a) Evaluate f(20) and interpret.

---

(b) Determine f′( x).

---

(c) Evaluate f′(20) and interpret.