Ch. 4 - Applications of the Derivative

Briggs - Calculus: Early Transcendentals 3rd Edition

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Briggs 3rd Edition

Ch. 4 - Applications of the Derivative

Problem 4.5.55a

Chapter 4, Problem 4.5.55a

Two poles of heights m and n are separated by a horizontal distance d. A rope is stretched from the top of one pole to the ground and then to the top of the other pole. Show that the configuration that requires the least amount of rope occurs when Θ₁ = Θ₂ (see figure). <IMAGE>

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Consider the two poles with heights m and n, separated by a horizontal distance d. Let the point where the rope touches the ground be P, dividing the distance d into two segments: x and (d-x).

The rope forms two right triangles. For the first triangle, with the pole of height m, the angle Θ₁ is formed between the rope and the ground. Similarly, for the second triangle, with the pole of height n, the angle Θ₂ is formed.

Using trigonometry, express the lengths of the rope segments in terms of x, m, n, and d. The length of the rope from the top of the first pole to the ground is given by the hypotenuse of the first triangle: √(x² + m²). Similarly, the length of the rope from the ground to the top of the second pole is: √((d-x)² + n²).

The total length of the rope is the sum of these two segments: L(x) = √(x² + m²) + √((d-x)² + n²). To minimize the total length of the rope, we need to find the value of x that minimizes L(x).

To find the minimum, take the derivative of L(x) with respect to x and set it to zero. This will give the condition for the minimum length. Solving this will show that the configuration with the least amount of rope occurs when Θ₁ = Θ₂, meaning the angles are equal, which is the condition for the minimum length of the rope.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Optimization

Optimization in calculus involves finding the maximum or minimum values of a function. In this context, we are looking to minimize the length of the rope stretched between two poles. This often requires setting up a function that represents the total length of the rope and then using techniques such as derivatives to find critical points where the length is minimized.

Trigonometric Functions

Trigonometric functions, such as sine and cosine, relate the angles of a triangle to the lengths of its sides. In this problem, the angles Θ₁ and Θ₂ are crucial for determining the lengths of the segments of the rope. Understanding how to express the lengths of these segments in terms of the angles will help in formulating the optimization problem.

Symmetry in Geometry

Symmetry in geometry refers to a situation where a configuration remains unchanged under certain transformations. In this problem, the configuration that minimizes the rope length occurs when the angles Θ₁ and Θ₂ are equal, indicating a symmetric arrangement. Recognizing this symmetry can simplify the analysis and lead to a more straightforward solution.

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