500
A Florida Citrus grower estimates that if 60 orange trees are planted; the average
yield per tree will be 400 oranges. The average yield will decrease by 4 oranges per tree for
each additional tree planted on the same acreage. How many trees should the grower plant to
maximize the total yield?
Let n= the number of additional trees. Let Y= the total yield = number of trees × the yield per tree.
Then:
Y (n) = (60trees + n · trees)(400oranges − n · 4oranges) = (60 + n)(400 − 4n) = 24, 000 + 160n − 4n^2
to maximize! Lets find the critical numbers:
Y'(n) = 160 − 8n = 0 ⇒ n =160/8 = 20 is the only critical number.
Moreover, Y"(n) = −8 ⇒ R"(20) = −8 < 0. By the second derivative test, Y has a local maximum at
n = 20, which is an absolute maximum since it is the only critical number.
The grower should plant 60+20 = 80 trees to maximize the total yield.