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Ch. 3 - Derivatives
Hass - Thomas' Calculus 15th Edition
Hass15th EditionThomas' CalculusISBN: 9780137616077Not the one you use?Change textbook
Chapter 3, Problem 3.95d

Right circular cylinder The total surface area S of a right circular cylinder is related to the base radius r and height h by the equation S = 2πr² + 2πrh.


d. How is dr/dt related to dh/dt if S is constant?

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Start by understanding the problem: We have a right circular cylinder with a constant total surface area S, and we need to find the relationship between the rates of change of the radius (dr/dt) and the height (dh/dt).
Given the surface area formula S = 2πr² + 2πrh, differentiate both sides with respect to time t. Since S is constant, its derivative with respect to time is zero.
Apply the chain rule to differentiate the right side: dS/dt = 0 = d/dt(2πr²) + d/dt(2πrh).
Differentiate each term: The derivative of 2πr² with respect to t is 4πr(dr/dt), and the derivative of 2πrh with respect to t is 2πr(dh/dt) + 2πh(dr/dt).
Set up the equation from the derivatives: 0 = 4πr(dr/dt) + 2πr(dh/dt) + 2πh(dr/dt). Solve this equation for dr/dt in terms of dh/dt, r, and h.

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Key Concepts

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

Implicit Differentiation

Implicit differentiation is a technique used to find the derivative of a function when it is not explicitly solved for one variable in terms of another. In this context, it allows us to differentiate the surface area equation with respect to time, treating both the radius and height as functions of time. This method is essential for relating the rates of change of the radius and height when the surface area is held constant.
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Finding The Implicit Derivative

Related Rates

Related rates involve finding the relationship between the rates of change of two or more variables that are related by an equation. In this problem, we are interested in how the rate of change of the radius (dr/dt) is connected to the rate of change of the height (dh/dt) while keeping the surface area constant. Understanding this concept is crucial for solving problems where multiple quantities change simultaneously.
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Intro To Related Rates

Surface Area of a Cylinder

The surface area of a right circular cylinder is given by the formula S = 2πr² + 2πrh, which includes both the area of the circular bases and the lateral surface area. This formula is fundamental to the problem as it establishes the relationship between the radius, height, and surface area. Recognizing how changes in r and h affect S is key to determining the relationship between dr/dt and dh/dt when S is constant.
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Example 1: Minimizing Surface Area
Related Practice
Textbook Question

Suppose that functions ƒ(x) and g(x) and their first derivatives have the following values at x = 0 and x = 1.


x ƒ(x) g(x) ƒ'(x) g'(x)

0 1 1 -3 1/2

1 3 5 1/2 -4


Find the first derivatives of the following combinations at the given value of x.


d. ƒ(g(x)), x = 0

Textbook Question

Analyzing Motion Using Graphs


[Technology Exercise] Exercises 31–34 give the position function s = f(t) of an object moving along the s-axis as a function of time t. Graph f together with the velocity function v(t) = ds/dt = f'(t) and the acceleration function a(t) = d²s/dt² = f''(t). Comment on the object’s behavior in relation to the signs and values of v and a. Include in your commentary such topics as the following:


d. When does it speed up and slow down?


s = t³ - 6t² + 7t, 0 ≤ t ≤ 4

Textbook Question

Theory and Examples


In Exercises 51–54,


c. For what values of x, if any, is f' positive? Zero? Negative?


y = −x²

Textbook Question

Theory and Examples


In Exercises 51–54,


d. Over what intervals of x-values, if any, does the function y = f(x) increase as x increases? Decrease as x increases? How is this related to what you found in part (c)? (We will say more about this relationship in Section 4.3.)


y = −1/x

Textbook Question

Theory and Examples


In Exercises 51–54,


d. Over what intervals of x-values, if any, does the function y = f(x) increase as x increases? Decrease as x increases? How is this related to what you found in part (c)? (We will say more about this relationship in Section 4.3.)


y = x³/3

Textbook Question

Average single-family home prices P (in thousands of dollars) in Sacramento, California, are shown in the accompanying figure from the beginning of 2006 through the end of 2015.



d. During what year did home prices drop most rapidly and what is an estimate of this rate?