/*! This file is auto-generated */ .wp-block-button__link{color:#fff;background-color:#32373c;border-radius:9999px;box-shadow:none;text-decoration:none;padding:calc(.667em + 2px) calc(1.333em + 2px);font-size:1.125em}.wp-block-file__button{background:#32373c;color:#fff;text-decoration:none} Problem 32 A right circular cylinder is ins... [FREE SOLUTION] | 91Ó°ÊÓ

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A right circular cylinder is inscribed in a cone with height \(h\) and base radius \(r .\) Find the largest possible volume of such a cylinder.

Short Answer

Expert verified
The largest volume is \(\frac{4}{27} \pi r^2 h\).

Step by step solution

01

Understand the Problem

We need to find the largest possible volume of a cylinder that fits inside a cone. The cone has known dimensions: height \(h\) and base radius \(r\). The cylinder must also be a right circular cylinder, meaning its sides are perpendicular to its bases.
02

Set Up Relations Between Cone and Cylinder

Visualize or draw a cross-section through the axis of symmetry of the cone and cylinder. The cross-section will be a triangle for the cone, and a rectangle for the cylinder. Let the height of the cylinder be \(H\) and its radius be \(R\). The base of this triangle is \(2r\) and its height is \(h\). The top surface of the cylinder will lie on a line connecting the vertex of the cone to the base of the cone in the cross-section triangle.
03

Express Cylinder Radius in terms of Cylinder Height

Using similar triangles, we find the relationship between the dimensions of the cylinder and the cone. The triangles formed BA and BC in the figure are similar, where one triangle has height \(h-H\) and base \(R\), while the whole cone has height \(h\) and base radius \(r\). This gives us the proportion \[ \frac{R}{r} = \frac{h-H}{h} \] Solving for \(R\), we get \[ R = r \frac{h-H}{h} \].
04

Write the Volume of the Cylinder as a Function of its Height

The volume \(V\) of the cylinder is given by \(V = \pi R^2 H\). Substitute the expression for \(R\) from Step 3, we have: \[ V = \pi \left( r \frac{h-H}{h} \right)^2 H = \pi r^2 \frac{(h-H)^2}{h^2} H \].
05

Optimize the Cylinder Volume Function

To find the maximum volume, we need to take the derivative of \(V\) with respect to \(H\) and set it zero to find critical points. First take the derivative:
06

Solve the Derivative Equation

Taking the derivative of \( V \) with respect to \( H \), we get \[ V'(H) = \pi r^2 \frac{d}{dH} \left( \frac{(h-H)^2}{h^2} H \right) \]. After differentiating and simplifying, we set \( V'(H) = 0 \) and solve for \( H \). This gives \( H = \frac{h}{3} \).
07

Find the Correspoding Radius \( R \)

Substitute \( H = \frac{h}{3} \) back into the expression for \( R \): \[ R = r \frac{h - \frac{h}{3}}{h} = r \frac{2}{3} \].
08

Calculate the Maximum Volume

Substitute \( H = \frac{h}{3} \, \text{and } \ R = \frac{2r}{3} \) into the volume formula: \[ V = \pi \left( \frac{2r}{3} \right)^2 \frac{h}{3} = \pi \frac{4r^2}{9} \frac{h}{3} = \pi \frac{4r^2 h}{27} \].
09

Conclusion

The largest possible volume of the cylinder inscribed in the cone is \( \frac{4}{27} \pi r^2 h \).

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Key Concepts

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

geometry of shapes
Geometry is all about understanding the sizes, shapes, and dimensions of different objects. In this optimization problem, the focus is on two shapes: a cone and a cylinder. A cone is a 3-dimensional shape with a circular base and a single vertex on the opposite side. When you draw a vertical cut through a cone, it resembles a triangle in geometry, which helps us to visualize spatial relationships. A cylinder, on the other hand, has two parallel circular bases connected by a curved surface. These shapes play a pivotal role in our problem, as the cylinder is inscribed within the cone. This means the cylinder fits perfectly inside the cone, touching the cone's interior sides. To solve this problem, it's important to understand how these two figures interact within 3-dimensional space, which involves not only looking at their individual geometric properties but also understanding how they fit together spatially.
derivatives
Derivatives are a fundamental tool in calculus. They represent the rate of change of a function concerning one of its variables. In optimization problems, we use derivatives to find the maximum or minimum values of a function. In this problem, the volume of the cylinder, which is a function of its height, needs to be maximized. The volume formula is expressed in terms of the cylinder's height, so by differentiating this function with respect to the height, we can find where the volume reaches its maximum.
The derivative helps us identify critical points by setting the derivative equal to zero. This helps us determine conditions under which the function neither increases nor decreases, thereby finding the optimal point, which, in this context, is the cylinder's maximum volume.
similar triangles
The concept of similar triangles is crucial in solving this optimization problem. Similar triangles have the same shape but can differ in size; they have equal corresponding angles and proportional corresponding side lengths.
In our exercise, the cone and the cylinder create a situation where similar triangles help set relationships between their dimensions. By examining a vertical cross-section of the cone, we can see that the ratio of the base radius of the cone to the height at any point is constant due to similarity. By applying the similar triangles principle, we determine how the radius of the cylinder changes as its height changes. This relationship is pivotal for setting up the equation that relates the cone's dimensions to the cylinder's dimensions, allowing us to express the cylinder's radius in terms of its height.
volume maximization
Volume maximization is the primary goal of this exercise. To maximize the volume of a cylinder within a cone, we need to express the volume \[ V = \pi R^2 H \] in terms of a single variable.
This involves substituting the expression for the radius found using similar triangles into the cylinder's volume formula. By optimizing the function through calculus, specifically by using its derivative, we identify conditions that maximize the cylinder's volume within the cone.
Finding this maximum volume involves calculating the derivative of the volume function, setting it to zero, and solving for the height, which reveals the dimensions that provide the largest volume possible. Understanding how these different mathematical concepts combine is key in solving this type of optimization problem.

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Most popular questions from this chapter

A woman at a point \(A\) on the shore of a circular lake with radius 2 mi wants to arrive at the point \(C\) diametrically opposite \(A\) on the other side of the lake in the shortest possible time (see the figure). She can walk at the rate of \(4 \mathrm{mi} / \mathrm{h}\) and row a boat at \(2 \mathrm{mi} / \mathrm{h}\). How should she proceed?

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