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Find the minimum surface area of a rectangular closed (top, bottom, and four sides) box with volume \(216 \mathrm{~m}^{3}\).

Short Answer

Expert verified
The minimum surface area is 216 m² with dimensions 6 m × 6 m × 6 m.

Step by step solution

01

Understand the Problem

The problem requires us to find the minimum surface area of a closed rectangular box with a fixed volume of 216 m³. This involves setting up equations for both the surface area and the volume of the box, and then finding the optimal dimensions that minimize the surface area.
02

Define Variables and Write Equations

Let the dimensions of the rectangular box be length ( l ), width ( w ), and height ( h ). The volume is given by:\[ V = l imes w imes h = 216 \]The surface area ( S ) of the box is given by:\[ S = 2lw + 2lh + 2wh \] Our task is to minimize this surface area while keeping the volume constant.
03

Express One Variable in Terms of Others

Using the volume equation, express h in terms of l and w :\[ h = \frac{216}{lw} \]Substitute h into the surface area formula to get:\[ S = 2lw + 2l \frac{216}{lw} + 2w \frac{216}{lw} \]
04

Simplify the Surface Area Formula

Substitute h and simplify the surface area expression:\[ S = 2lw + \frac{432}{w} + \frac{432}{l} \]This is the expression to minimize with respect to l and w .
05

Use Calculus to Find the Minimum

To find the minimum surface area, we'll take partial derivatives with respect to l and w , set them to zero, and solve for l and w .\[ \frac{\partial S}{\partial l} = 2w - \frac{432}{l^2} = 0 \]\[ \frac{\partial S}{\partial w} = 2l - \frac{432}{w^2} = 0 \]Solve these equations to find l and w.
06

Solve for Optimal Dimensions

From the equations:\[ 2w = \frac{432}{l^2} \quad \Rightarrow \quad w = \frac{216}{l^2} \]\[ 2l = \frac{432}{w^2} \quad \Rightarrow \quad l = \frac{216}{w^2} \]From the symmetry and solving, we can deduce l = w = h = 6 m as they satisfy both conditions.
07

Verify the Solution

Check the dimensions satisfy the volume constraint:\[ 6 \times 6 \times 6 = 216 \]Calculate the surface area using l = w = h = 6 m:\[ S = 2(6 \times 6) + 2(6 \times 6) + 2(6 \times 6) = 216 \]This is indeed a minimum for the surface area.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Surface Area Minimization
Finding the minimum surface area of an object, such as a box, requires us to apply specific mathematical techniques. The primary goal here is to determine the dimensions that will result in the smallest possible surface area while maintaining a given volume. For a box, the surface area formula incorporates the dimensions: length ( l ), width ( w ), and height ( h ). This required calculating the total area covered, including both the top and bottom, as well as the four sides.
Utilizing the given volume constraint, you maintain realistic proportions. This involves expressing one variable in terms of others, allowing you to effectively reduce the number of variables involved. Hence, we express h in terms of l and w to simplify calculations. Ultimately, this approach aims to simplify the original surface area equation, thereby making it easier to minimize.
Partial Derivatives
Partial derivatives are an essential tool in calculus, especially when dealing with functions that depend on multiple variables. They allow us to measure how a function changes as one specific variable changes, while holding the others constant. In context, the surface area of the box, represented as a function of l and w , is differentiated with respect to each variable.
This means we're separately looking at how changes in l and w impact the surface area. By taking the partial derivatives, setting them to zero, and solving, you can identify critical points where the surface area may be at its minimum or maximum. It's the equivalent of looking at the slope of the function in a specific direction - if the slope is zero, the function might be reaching a turning point. Calculating these derivatives helps pinpoint the optimal dimensions that minimize the surface area.
Constraint Optimization
Constraint optimization is a process of finding the best solution given specific limitations or requirements. Here, we're dealing with both a surface area to minimize and a set volume of 216 m^3 to maintain. In mathematical terms, the volume acts as a constraint, which limits the dimensions in finding the surface area.
To handle this, you first express one variable in terms of others using the constraint (e.g., using the volume equation). Then you integrate this transformed variable back into the equation you’re trying to optimize. This technique is essential because it reduces the effective number of variables, thus simplifying the problem.
Once integrated, employing calculus, particularly derivative techniques, helps to zero in on the set of dimensions that yield the desired objective - the minimum surface area in this case. Throughout this process, it’s crucial to ensure that any proposed solutions satisfy the original constraints such as rounded dimensions and checking if the conditions are indeed a minimum.

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