David has 400 yards of fencing and wishes to enclose a rectangular area against a pre-existing fence. What equation should David use to help him maximize the area?

A. f(x)=x(400-2x)
B. f(x)=x(400+2x)
C. f(x)=2x(400-x)
D. f(x)=2x(400+x)


Answer: f(x)=x(400-2x)

Respuesta :

The equation for the area in terms of x gives the function David should

use to maximize the area.

Response:

The equation that David should use to help him maximize the area is option A.

  • A. f(x) = x·(400 - 2·x)

How can the function for the area be obtained?

The given parameter are;

Length of fencing David has = 400 yards

Area to be enclosed = Rectangular area with pre-existing fence (on one side)

Required:

The equation David should use to maximize the area.

Solution:

Let x represent the width of the area (the sides perpendicular to the pre-

existing fence), and let y represent the length of pre-existing fence

We have;

Length of fencing available = Opposite side to pre-existing fence + 2 × Width of the area

Opposite side to pre-existing fence = Length of pre-existing fence

Which gives;

Length of fencing available = Length of pre-existing fence + 2 × Width of the area

400 = y + 2·x

y = 400 - 2·x

Area = Length × Width

Which gives;

Area = y × x

Area as a function of x is therefore;

Area, f(x)= (400 - 2·x) × x

Which gives;

  • f(x) = x·(400 - 2·x)

The correct option is therefore;

  • A. f(x) = x·(400 - 2·x)

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