To determine the domain, we need to analyze the situation specified in the problem.
In this case, Marilee can build a rectangular enclosure using the fencing materials, with the perimeter given by \( P = 2l + 2w \), where \( l \) is the length and \( w \) is the width. Given that she has 40 yards of fencing, we can write the equation for the perimeter as:
\[
2l + 2w = 40 \implies l + w = 20 \implies l = 20 - w
\]
The area \( A \) of the rectangle is given by:
\[
A = l \cdot w = (20 - w) \cdot w = 20w - w^2
\]
This simplifies to the quadratic equation provided in the problem:
\[
A = -w^2 + 20w
\]
To find the domain of this function, we need to consider the physical constraints on the width \( w \):
- The width \( w \) must be non-negative: \( w \geq 0 \)
- The total width and length must not exceed 20 yards, as \( w + l = 20 \): \( w \leq 20 \)
Combining these two constraints, the domain for \( w \) is:
\[
0 \leq w \leq 20
\]
Now, you are asking about the maximum range when the maximum area is achieved. The maximum area occurs at the vertex of the quadratic equation. The \( w \)-coordinate of the vertex can be found using the formula \( w = -\frac{b}{2a} \), where \( A = aw^2 + bw + c \).
In our case, \( a = -1 \) and \( b = 20 \):
\[
w = -\frac{20}{2 \cdot -1} = 10
\]
At \( w = 10 \), we can find the corresponding area:
\[
A(10) = -10^2 + 20 \cdot 10 = -100 + 200 = 100 \text{ square yards}
\]
The maximum area (range) of the garden is 100 square yards when the width is 10 yards.
Therefore, the domain we're discussing refers specifically to the width, which has a maximum of 20 yards, but the answer to your question about the maximum range achieved is correlated to:
10 yards (the width at which the maximum area is achieved).