Question
You plan to enclosed part of a rectangular farmland with a fence. Since one side of it is bounded by a river, you only need to fence the other three sides. if you have enough budget to buy 600m of fencing material, what is the largest area you can enclose?
Answers
Reiny
side parallel to river --- y m
each of other two sides --- x
2x + y = 600 ---> y = 600-2x
area = xy
= x(600-2x) = -2x^2 + 600x
d(area)/dx = -4x + 600
= 0 for a max of area
-4x = -600
x = 150
then y = 600-2x = 300
largest area = 150(300) = 45000 m^2
each of other two sides --- x
2x + y = 600 ---> y = 600-2x
area = xy
= x(600-2x) = -2x^2 + 600x
d(area)/dx = -4x + 600
= 0 for a max of area
-4x = -600
x = 150
then y = 600-2x = 300
largest area = 150(300) = 45000 m^2