Math Problem Statement

A right circular cone is to be inscribed in another right circular cone of volume 6m3 and altitude 7m, with the same axis and with the vertex of the inner cone touching the base of the outer cone. What must be the altitude of the inscribed cone in order to have the the largest possible volume?

Solution

To solve this problem, we aim to find the altitude of the inscribed cone that maximizes its volume. Let's break it down step by step:


1. Outer cone specifications:

  • Volume: Vouter=6m3V_{\text{outer}} = 6 \, \text{m}^3
  • Altitude: houter=7mh_{\text{outer}} = 7 \, \text{m}

From the volume formula for a cone, V=13πr2hV = \frac{1}{3} \pi r^2 h, the radius of the base of the outer cone is:

Vouter=13πrouter2houterV_{\text{outer}} = \frac{1}{3} \pi r_{\text{outer}}^2 h_{\text{outer}} 6=13πrouter2(7)6 = \frac{1}{3} \pi r_{\text{outer}}^2 (7) router2=187π    router=187πr_{\text{outer}}^2 = \frac{18}{7 \pi} \implies r_{\text{outer}} = \sqrt{\frac{18}{7 \pi}}


2. Geometry of the inscribed cone:

The inscribed cone shares the same axis with the outer cone, and its vertex is on the base of the outer cone. If the altitude of the inscribed cone is hh, the radius of its base is proportional to its altitude because the cones are similar in shape. Let the ratio of similarity be kk:

k=rinscribedrouter=hhouter=h7k = \frac{r_{\text{inscribed}}}{r_{\text{outer}}} = \frac{h}{h_{\text{outer}}} = \frac{h}{7}

Thus, the radius of the inscribed cone is: rinscribed=krouter=h7routerr_{\text{inscribed}} = k \cdot r_{\text{outer}} = \frac{h}{7} \cdot r_{\text{outer}}


3. Volume of the inscribed cone:

The volume VinscribedV_{\text{inscribed}} of the inscribed cone is: Vinscribed=13πrinscribed2hV_{\text{inscribed}} = \frac{1}{3} \pi r_{\text{inscribed}}^2 h Substitute rinscribed=h7routerr_{\text{inscribed}} = \frac{h}{7} \cdot r_{\text{outer}}: Vinscribed=13π(h7router)2hV_{\text{inscribed}} = \frac{1}{3} \pi \left( \frac{h}{7} \cdot r_{\text{outer}} \right)^2 h Vinscribed=13π(h7)2router2hV_{\text{inscribed}} = \frac{1}{3} \pi \left( \frac{h}{7} \right)^2 r_{\text{outer}}^2 h Vinscribed=13πh349router2V_{\text{inscribed}} = \frac{1}{3} \pi \frac{h^3}{49} r_{\text{outer}}^2

Substitute router2=187πr_{\text{outer}}^2 = \frac{18}{7 \pi}: Vinscribed=13πh349187πV_{\text{inscribed}} = \frac{1}{3} \pi \frac{h^3}{49} \cdot \frac{18}{7 \pi} Vinscribed=6343h3V_{\text{inscribed}} = \frac{6}{343} h^3


4. Maximizing VinscribedV_{\text{inscribed}}:

To maximize VinscribedV_{\text{inscribed}}, differentiate it with respect to hh and set the derivative to zero: Vinscribed=6343h3V_{\text{inscribed}} = \frac{6}{343} h^3 dVinscribeddh=18343h2\frac{dV_{\text{inscribed}}}{dh} = \frac{18}{343} h^2 Set dVinscribeddh=0\frac{dV_{\text{inscribed}}}{dh} = 0: 18343h2=0    h=0 or h=7\frac{18}{343} h^2 = 0 \implies h = 0 \text{ or } h = 7

For the volume to be maximized within the cone, hh cannot be 00. The maximum occurs at h=7mh = 7 \, \text{m}, but h=7h = 7 corresponds to the outer cone. To ensure the cone is inscribed, let's confirm h<7h < 7.

5. Confirming Geometry and Limits

Actually, I realized a small misstep. Let's revisit and properly conclude the condition.


Since hh must be less than 7 (as the inscribed cone cannot extend beyond the outer cone), the critical step is in correctly setting up our optimization problem using geometry and derivatives.

For simplicity:

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Math Problem Analysis

Mathematical Concepts

Geometry
Optimization
Volume of a Cone
Similarity of Shapes

Formulas

Volume of a cone: V = (1/3)πr²h
Similarity ratio: k = h_inner / h_outer
Maximization using derivatives

Theorems

Similarity of triangles
Calculus-based optimization

Suitable Grade Level

Grades 11-12 (Pre-Calculus/Calculus)