The psycologist L. L thurstone studied learning by asking subjects to memorize a list (n) words long. One formula he used to predict learning time,...

The psycologist L. L thurstone studied learning by asking subjects to memorize a list (n) words long. One formula he used to predict learning time, T, was T=4n(n-3)^1/2. Find the rate of change of learning time with respect to the length of the list. What is the rate of change in time when there are 19 words on the list?