You need to make a fence that boarders a cliff. It must be rectangular in shape and you are asked to use the cliff as one of the sides (so only 3 sides of fencing). The fencing costs $5 per foot. If the area of the rectangular enclosure must be 10,000 square feet, what must the dimensions be in order to minimize the cost?

Calculus: Early Transcendentals
8th Edition
ISBN:9781285741550
Author:James Stewart
Publisher:James Stewart
Chapter1: Functions And Models
Section: Chapter Questions
Problem 1RCC: (a) What is a function? What are its domain and range? (b) What is the graph of a function? (c) How...
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**Problem Statement:**

You need to make a fence that borders a cliff. It must be rectangular in shape and you are asked to use the cliff as one of the sides (so only 3 sides of fencing). The fencing costs $5 per foot. If the area of the rectangular enclosure must be 10,000 square feet, what must the dimensions be in order to minimize the cost?

**Explanation:**

In this problem, we want to find the dimensions of the rectangle with one side as the cliff. The enclosure consists of a width and two lengths. To minimize the cost, we need to minimize the total length of fencing used, considering the cost per foot.

**Approach to Solution:**

1. **Define Variables:**
   - Let \( x \) be the width (the side perpendicular to the cliff).
   - Let \( y \) be the length (parallel to the cliff).

2. **Area Constraint:**
   - The area of the rectangle is given by \( x \times y = 10,000 \).

3. **Cost Function:**
   - The cost to fence the three sides is proportional to their length: \( C = 5 \times (2x + y) \).
   - Substitute \( y \) from the area constraint: \( y = \frac{10,000}{x} \).
   - Thus, the cost function becomes: \( C(x) = 5 \times (2x + \frac{10,000}{x}) \).

4. **Optimization:**
   - Differentiate \( C(x) \) to find the critical points, then solve for \( x \).
   - Use these critical points to determine the dimension \( x \) that minimizes the fencing cost and compute \( y \) accordingly.
Transcribed Image Text:**Problem Statement:** You need to make a fence that borders a cliff. It must be rectangular in shape and you are asked to use the cliff as one of the sides (so only 3 sides of fencing). The fencing costs $5 per foot. If the area of the rectangular enclosure must be 10,000 square feet, what must the dimensions be in order to minimize the cost? **Explanation:** In this problem, we want to find the dimensions of the rectangle with one side as the cliff. The enclosure consists of a width and two lengths. To minimize the cost, we need to minimize the total length of fencing used, considering the cost per foot. **Approach to Solution:** 1. **Define Variables:** - Let \( x \) be the width (the side perpendicular to the cliff). - Let \( y \) be the length (parallel to the cliff). 2. **Area Constraint:** - The area of the rectangle is given by \( x \times y = 10,000 \). 3. **Cost Function:** - The cost to fence the three sides is proportional to their length: \( C = 5 \times (2x + y) \). - Substitute \( y \) from the area constraint: \( y = \frac{10,000}{x} \). - Thus, the cost function becomes: \( C(x) = 5 \times (2x + \frac{10,000}{x}) \). 4. **Optimization:** - Differentiate \( C(x) \) to find the critical points, then solve for \( x \). - Use these critical points to determine the dimension \( x \) that minimizes the fencing cost and compute \( y \) accordingly.
Expert Solution
Step 1

Let x feet be the length of the side of the rectangle that is parallel to the cliff and y feet be the length of the other side.

Area of the rectangular enclosure is 10,000 square feet.

xy=10000y=10000x 

Step 2

Cost of fencing is

C=5x+2yC=5x+10yC=5x+100,000x

Now, differentiate both sides with respect to x.

C'(x)=5-100,000x2

Now, we solve the equation

C'(x)=05-100,000x2=0x2=20,000x=1002

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