Math Problem Statement

The volume of a cube is decreasing at a constant rate of 1334 cubic centimeters per second. At the instant when the side length of the cube is 7 centimeters, 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, let’s break it down into steps.

  1. Define the volume and surface area of a cube in terms of its side length:

    • Let ss be the side length of the cube (in cm).
    • The volume VV of a cube is given by: V=s3V = s^3
    • The surface area AA of a cube is given by: A=6s2A = 6s^2
  2. Differentiate volume with respect to time:

    Since the volume is decreasing, dVdt=1334cm3/s\frac{dV}{dt} = -1334 \, \text{cm}^3/\text{s}.

    Now, using the formula V=s3V = s^3, differentiate both sides with respect to time tt: dVdt=3s2dsdt\frac{dV}{dt} = 3s^2 \frac{ds}{dt}

    Substitute dVdt=1334\frac{dV}{dt} = -1334 and s=7s = 7 cm: 1334=372dsdt-1334 = 3 \cdot 7^2 \cdot \frac{ds}{dt} 1334=349dsdt-1334 = 3 \cdot 49 \cdot \frac{ds}{dt} 1334=147dsdt-1334 = 147 \cdot \frac{ds}{dt} Solving for dsdt\frac{ds}{dt}: dsdt=13341479.0748cm/s\frac{ds}{dt} = \frac{-1334}{147} \approx -9.0748 \, \text{cm/s}

  3. Differentiate surface area with respect to time:

    Now, differentiate A=6s2A = 6s^2 with respect to tt to find dAdt\frac{dA}{dt}: dAdt=12sdsdt\frac{dA}{dt} = 12s \cdot \frac{ds}{dt}

    Substitute s=7s = 7 cm and dsdt9.0748\frac{ds}{dt} \approx -9.0748: dAdt=127(9.0748)\frac{dA}{dt} = 12 \cdot 7 \cdot (-9.0748) dAdt1279.0748\frac{dA}{dt} \approx 12 \cdot 7 \cdot -9.0748 dAdt762.2864\frac{dA}{dt} \approx -762.2864 Rounded to three decimal places, the rate of change of the surface area is: dAdt762.286cm2/s\frac{dA}{dt} \approx -762.286 \, \text{cm}^2/\text{s}

So, the rate of change of the surface area at the instant when the side length is 7 cm is approximately 762.286-762.286 square centimeters per second.

Would you like more details or have any questions?


Here are five related questions to expand understanding:

  1. How would the rate of change in surface area vary if the side length of the cube was larger?
  2. If the side length of the cube was increasing instead, how would that affect the surface area rate?
  3. How is the rate of change of the side length related to the rate of change of the volume?
  4. How would this problem differ if the shape was a rectangular prism rather than a cube?
  5. Can the rate of surface area change ever exceed the rate of volume change?

Tip: Always check the units of rates carefully, especially when dealing with derivatives involving different powers like surface area (square units) and volume (cubic units).

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Math Problem Analysis

Mathematical Concepts

Differentiation
Related Rates
Geometry of 3D Shapes

Formulas

Volume of a cube V = s^3
Surface area of a cube A = 6s^2
Differentiation with respect to time: dV/dt and dA/dt

Theorems

Chain Rule for Differentiation

Suitable Grade Level

Grades 11-12