Respuesta :
y = 220/ ((10/3) * √22)
It is given that area is 200 square feet. Cost is 3 dollar per foot. Fourth side costs 12 dollars per foot.
This f(x,y) needs to represent the cost of the fence so we can look at each side's price. We can choose the fourth side to be along the x axis and be represented by 13x. The other sides therefore must be represented by 5y, 5y, and 5x.
So, our cost, f(x,y) = 13x + 5x + 5y + 5y = 18x + 10y
Our constraint is xy = 220 as defined by the area of a rectangle.
We can then take our constraint to be put in terms of exclusively x, giving us y = 220/x
Plugging this into our cost, the thing we are minimizing, we get f(x) = 18x + 2200/x
In order to find the minimum we use the first derivative test, taking f'(x) and finding the critical points.
f'(x) = 18 - 2200/x2
Setting this equation to be equal to 0 we find that x = ±√2200/18 . But the negative answer doesn't make sense because distance cannot be negative so we throw it out.
x = (10/3) * √22
We must verify that this is a minimum by confirming the following:
If x < (10/3) * √22, f'(x) < 0. So, f(x) is decreasing when x < (10/3) * √22.
If x > (10/3) * √22, f'(x) > 0. So, f(x) is increasing when x > (10/3) * √22.
Thus, we have guaranteed that (10/3) * √22 is the x dimension. Now we plug in this value in our original equation to find y and that is our y dimension. So, y = 220/ ((10/3) * √22)
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