11.4 (0) You are the business manager of P. Bunyan Forests, Inc., and
are trying to decide when you should cut your trees. The market value of
the lumber that you will get if you let your trees reach the age of t years
is given by the function W(t) = e^(0.20t-0.001t^2). Mr. Bunyan can earn an
interest rate of 5% per year on money in the bank.
The rate of growth of the market value of the trees will be greater
than 5% until the trees reach  ________ years of age. (Hint: It follows
from elementarycalcu lus that if F(t) = eg(t), then F(t)/F(t) = g(t).)
(a) If he is only interested in the trees as an investment, how old should
Mr. Bunyan let the trees get? __________years.
(b) At what age do the trees have the greatest market value? ________years

这是现代观点练习册中的一道题。给出的答案分别是：75，75，100。我就是不明白，为什么最后那个空是100years。我觉得应该是75啊