Math Problem Statement

The volume of a cube is decreasing at a constant rate of 1699 cubic meters per second. At the instant when the volume of the cube is 629 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

To solve this problem, we need to relate the volume and surface area of the cube, and then find how the rate of change of the volume affects the rate of change of the surface area.

Step 1: Relationships between volume and side length

Let the side length of the cube be ss. The volume VV and surface area AA of the cube are given by:

  • Volume: V=s3V = s^3
  • Surface area: A=6s2A = 6s^2

We are given that the volume is decreasing at a constant rate: dVdt=1699 cubic meters per second\frac{dV}{dt} = -1699 \text{ cubic meters per second}

We are also given that at the instant of interest, the volume is: V=629 cubic metersV = 629 \text{ cubic meters}

Step 2: Differentiate the volume and surface area equations

To find the rate of change of the surface area, we will differentiate both the volume and surface area equations with respect to time tt.

  1. Differentiating V=s3V = s^3: dVdt=3s2dsdt\frac{dV}{dt} = 3s^2 \frac{ds}{dt}

  2. Differentiating A=6s2A = 6s^2: dAdt=12sdsdt\frac{dA}{dt} = 12s \frac{ds}{dt}

Step 3: Solve for dsdt\frac{ds}{dt}

From the first equation, we can solve for dsdt\frac{ds}{dt}: dVdt=3s2dsdt\frac{dV}{dt} = 3s^2 \frac{ds}{dt} 1699=3s2dsdt-1699 = 3s^2 \frac{ds}{dt} dsdt=16993s2\frac{ds}{dt} = \frac{-1699}{3s^2}

Step 4: Find ss when V=629V = 629

Since V=s3V = s^3, we can solve for ss: s3=629s^3 = 629 s=6293s = \sqrt[3]{629}

Let’s calculate ss:

s8.559 meterss \approx 8.559 \text{ meters}

Step 5: Calculate dsdt\frac{ds}{dt}

Now that we have ss, we can substitute it into the equation for dsdt\frac{ds}{dt}: dsdt=16993(8.559)2\frac{ds}{dt} = \frac{-1699}{3(8.559)^2}

Let’s calculate dsdt\frac{ds}{dt}:

dsdt16993×73.236=1699219.708\frac{ds}{dt} \approx \frac{-1699}{3 \times 73.236} = \frac{-1699}{219.708} dsdt7.735 meters per second\frac{ds}{dt} \approx -7.735 \text{ meters per second}

Step 6: Calculate dAdt\frac{dA}{dt}

Finally, we substitute ss and dsdt\frac{ds}{dt} into the equation for dAdt\frac{dA}{dt}: dAdt=12sdsdt\frac{dA}{dt} = 12s \frac{ds}{dt} dAdt=12(8.559)(7.735)\frac{dA}{dt} = 12(8.559)(-7.735) dAdt794.825 square meters per second\frac{dA}{dt} \approx -794.825 \text{ square meters per second}

Final Answer:

The rate of change of the surface area of the cube is approximately: 794.825 square meters per second.\boxed{-794.825} \text{ square meters per second}.

Let me know if you'd like further details or clarifications!


Here are 5 related questions:

  1. How would the solution change if the volume were increasing instead of decreasing?
  2. What is the relationship between the side length and surface area in terms of rates of change?
  3. How does the surface area change if the volume remains constant?
  4. Can the rate of volume change ever directly equal the rate of surface area change?
  5. How would this problem change if we were working with a rectangular prism instead of a cube?

Tip: Remember that volume changes scale with the cube of the side length, while surface area changes scale with the square of the side length.

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Math Problem Analysis

Mathematical Concepts

Geometry
Differentiation
Related Rates

Formulas

Volume of a cube: V = s^3
Surface area of a cube: A = 6s^2
Related rates formula for volume: dV/dt = 3s^2 * ds/dt
Related rates formula for surface area: dA/dt = 12s * ds/dt

Theorems

Chain rule for differentiation
Relationship between volume and surface area of a cube

Suitable Grade Level

Grades 11-12 (Advanced Calculus)