Math Problem Statement

Solve

The volume of a cube is decreasing at a constant rate of 511 cubic meters per minute. At the instant when the volume of the cube is 186 cubic meters, what is the rate of change of the surface area of the cube? Round your answer to three decimal places (if necessary).

Solution

Let's solve the problem step by step.


Given:

  1. Volume of a cube V=s3V = s^3, where ss is the side length of the cube.
  2. dVdt=511m3/min\frac{dV}{dt} = -511 \, \text{m}^3/\text{min} (volume is decreasing at 511 cubic meters per minute).
  3. V=186m3V = 186 \, \text{m}^3 (volume at the instant of interest).
  4. Find dAdt\frac{dA}{dt}, the rate of change of the surface area, where A=6s2A = 6s^2.

Step 1: Relating volume and side length

The volume of a cube is given by: V=s3V = s^3 Solving for ss: s=V3s = \sqrt[3]{V} At V=186m3V = 186 \, \text{m}^3: s=18635.729ms = \sqrt[3]{186} \approx 5.729 \, \text{m}


Step 2: Differentiating volume with respect to time

Differentiating V=s3V = s^3 with respect to tt: dVdt=3s2dsdt\frac{dV}{dt} = 3s^2 \frac{ds}{dt} Substituting dVdt=511\frac{dV}{dt} = -511: 511=3s2dsdt-511 = 3s^2 \frac{ds}{dt} At s5.729s \approx 5.729: 511=3(5.729)2dsdt-511 = 3(5.729)^2 \frac{ds}{dt} Calculating s2s^2: 5.729232.8265.729^2 \approx 32.826 511=3(32.826)dsdt-511 = 3(32.826) \frac{ds}{dt} 511=98.478dsdt-511 = 98.478 \frac{ds}{dt} dsdt=51198.4785.191m/min\frac{ds}{dt} = \frac{-511}{98.478} \approx -5.191 \, \text{m/min}


Step 3: Differentiating surface area with respect to time

The surface area of the cube is: A=6s2A = 6s^2 Differentiating with respect to tt: dAdt=12sdsdt\frac{dA}{dt} = 12s \frac{ds}{dt} Substitute s5.729s \approx 5.729 and dsdt5.191\frac{ds}{dt} \approx -5.191: dAdt=12(5.729)(5.191)\frac{dA}{dt} = 12(5.729)(-5.191) dAdt12(5.729)(5.191)356.752m2/min\frac{dA}{dt} \approx 12(5.729)(-5.191) \approx -356.752 \, \text{m}^2/\text{min}


Final Answer:

The rate of change of the surface area is approximately: 356.752m2/min\boxed{-356.752 \, \text{m}^2/\text{min}}

Let me know if you need further clarification or additional steps!


5 Related Questions:

  1. What is the relationship between surface area and volume for a cube?
  2. How does the rate of change of volume relate to the rate of change of side length for a cube?
  3. How would the problem change if the cube's volume were increasing instead?
  4. How can you compute the side length directly using logarithms for more precision?
  5. Can you derive a general formula for dAdt\frac{dA}{dt} in terms of dVdt\frac{dV}{dt} without calculating ss?

Tip:

For cubes, always link volume, surface area, and side length using their geometric formulas before differentiating!

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Math Problem Analysis

Mathematical Concepts

Geometry
Differentiation
Related Rates

Formulas

V = s^3 (Volume of a cube)
A = 6s^2 (Surface area of a cube)
dV/dt = 3s^2 ds/dt (Rate of change of volume)
dA/dt = 12s ds/dt (Rate of change of surface area)

Theorems

Chain Rule of Differentiation

Suitable Grade Level

Grades 11-12 or early college (Calculus)