Chapter 3: Problem 325
The volume of a right circular cone of radius \(x\) and height \(y\) is given by \(V=\frac{1}{3} \pi x^{2} y .\) Suppose that the volume of the cone is 85\(\pi \mathrm{cm}^{3} .\) Find \(\frac{d y}{d x}\) when \(x=4\) and \(y=16\)
Short Answer
Expert verified
\( \frac{dy}{dx} = -8 \)
Step by step solution
01
Understand the Given Information
The volume of a cone is given by the formula \( V = \frac{1}{3} \pi x^2 y \). We know the volume is \( 85\pi \) cm\(^3\), therefore, \( \frac{1}{3} \pi x^2 y = 85\pi \). We need to find \( \frac{dy}{dx} \) when \( x = 4 \) and \( y = 16 \).
02
Simplify the Equation
First, eliminate \( \pi \) from the equation since it appears on both sides: \( \frac{1}{3} x^2 y = 85 \). Next, multiply through by 3 to clear the fraction: \( x^2 y = 255 \).
03
Differentiate with respect to \( x \)
Differentiate both sides of \( x^2 y = 255 \) with respect to \( x \), applying the product rule to the left side: \( \frac{d}{dx}(x^2 y) = 2x y + x^2 \frac{dy}{dx} = 0 \) because the derivative of a constant is zero.
04
Solve for \( \frac{dy}{dx} \)
Rearrange the differentiated equation to solve for \( \frac{dy}{dx} \): \( x^2 \frac{dy}{dx} = -2xy \). Thus, \( \frac{dy}{dx} = \frac{-2xy}{x^2} = \frac{-2y}{x} \).
05
Substitute \( x = 4 \) and \( y = 16 \) into the Derivative
Substitute the given values into the equation: \( \frac{dy}{dx} = \frac{-2(16)}{4} = \frac{-32}{4} = -8 \).
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Volume of a Cone
The volume of a cone is a fundamental concept in geometry. Cones are three-dimensional shapes with a circular base that narrows to a point called the apex. The volume of a cone is calculated using the formula:
- \( V = \frac{1}{3} \pi x^2 y \)
Product Rule
When dealing with differentiation of equations involving products of variables, the product rule is essential. For a function \( f \cdot g \), where both \( f \) and \( g \) depend on \( x \), the derivative is given by:
- \( \frac{d}{dx}(f \cdot g) = \frac{df}{dx} \cdot g + f \cdot \frac{dg}{dx} \)
Chain Rule
The chain rule helps us differentiate composite functions by breaking them down into simpler parts. If we have an expression like \( h(g(x)) \), the chain rule states:
- \( \frac{dh}{dx} = \frac{dh}{dg} \cdot \frac{dg}{dx} \)
Derivatives
Derivatives measure how a function changes as its input changes. They're a cornerstone of calculus, often represented as \( \frac{dy}{dx} \) or \( f'(x) \). The calculation of derivatives provides the rate at which one quantity changes with respect to another. This exercise involves finding the derivative \( \frac{dy}{dx} \), which indicates how the height \( y \) of a cone changes as its radius \( x \) changes, given a constant volume. By differentiating the volume equation, we obtain relations like \( \frac{-2y}{x} \), which specifies the precise rate of change. Mastery of derivatives and their applications is vital for tackling dynamic changes in mathematical and real-world scenarios.