ny fixed number n representing the perimeter of a rectangle, what are the dimensions of ectangle that maximizes the area of that rectangle?

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ISBN:9780470458365
Author:Erwin Kreyszig
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What could one do to solve this optimization problem?

**Question 8:**

For any fixed number \( n \) representing the perimeter of a rectangle, what are the dimensions of the rectangle that maximizes the area of that rectangle?

**Explanation:**

To find the dimensions of the rectangle that maximizes the area given a fixed perimeter \( n \), we need to use the formula for the perimeter of a rectangle, \( P = 2(l + w) \), where \( l \) is the length and \( w \) is the width. 

Rearranging the formula gives \( l + w = \frac{n}{2} \).

The area \( A \) of a rectangle is given by \( A = l \times w \).

To maximize the area, substitute \( w = \frac{n}{2} - l \) into the area formula:

\[ A = l \times \left(\frac{n}{2} - l\right) \]
\[ A = \frac{n}{2}l - l^2 \]

This quadratic equation can be simplified as:

\[ A = -l^2 + \frac{n}{2}l \]

To find the value of \( l \) that maximizes the area, we take the derivative of \( A \) with respect to \( l \) and set it to zero:

\[ \frac{dA}{dl} = -2l + \frac{n}{2} = 0 \]

Solving for \( l \), we get:

\[ l = \frac{n}{4} \]

Since \( l = w \) when the area is maximized, the rectangle becomes a square. Therefore, the dimensions of the rectangle that maximize the area are \( \frac{n}{4} \times \frac{n}{4} \).

**Conclusion:**

For a given perimeter \( n \), the dimensions of the rectangle that maximize the area are those of a square with sides each measuring \( \frac{n}{4} \).
Transcribed Image Text:**Question 8:** For any fixed number \( n \) representing the perimeter of a rectangle, what are the dimensions of the rectangle that maximizes the area of that rectangle? **Explanation:** To find the dimensions of the rectangle that maximizes the area given a fixed perimeter \( n \), we need to use the formula for the perimeter of a rectangle, \( P = 2(l + w) \), where \( l \) is the length and \( w \) is the width. Rearranging the formula gives \( l + w = \frac{n}{2} \). The area \( A \) of a rectangle is given by \( A = l \times w \). To maximize the area, substitute \( w = \frac{n}{2} - l \) into the area formula: \[ A = l \times \left(\frac{n}{2} - l\right) \] \[ A = \frac{n}{2}l - l^2 \] This quadratic equation can be simplified as: \[ A = -l^2 + \frac{n}{2}l \] To find the value of \( l \) that maximizes the area, we take the derivative of \( A \) with respect to \( l \) and set it to zero: \[ \frac{dA}{dl} = -2l + \frac{n}{2} = 0 \] Solving for \( l \), we get: \[ l = \frac{n}{4} \] Since \( l = w \) when the area is maximized, the rectangle becomes a square. Therefore, the dimensions of the rectangle that maximize the area are \( \frac{n}{4} \times \frac{n}{4} \). **Conclusion:** For a given perimeter \( n \), the dimensions of the rectangle that maximize the area are those of a square with sides each measuring \( \frac{n}{4} \).
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