Answer:
390 m (perpendicular to river) x 780 m (parallel to river)
Step-by-step explanation:
Let y be the length of the side parallel to the river, and let x be the length of the sides perpendicular to the river.
The total area and length of fence required are given by:
Rewriting the length of fence as a function of only x:
The value of x for which the derivate of L(x) is zero is the length of x that uses the least amount of fencing:
If x = 390 m, then:
The dimensions that will use the least amount of fencing are 390 m x 780 m