Math Problem Statement

The volume of a cube is increasing at the rate of 6cm3/s. How fast is the surface 2 area of cube increasing, when the length of an edge is cm?

Solution

To solve this, we need to use related rates involving the volume and surface area of a cube. Let xx be the length of an edge of the cube (in cm), VV the volume, and AA the surface area.

Step 1: Relations for Volume and Surface Area

  1. The volume of a cube is given by: V=x3V = x^3
  2. The surface area of a cube is given by: A=6x2A = 6x^2

Step 2: Differentiating Volume and Surface Area with Respect to Time

  1. Differentiate the volume equation V=x3V = x^3 with respect to time tt: dVdt=3x2dxdt\frac{dV}{dt} = 3x^2 \frac{dx}{dt} We are given that dVdt=6cm3/s\frac{dV}{dt} = 6 \, \text{cm}^3/\text{s}.

  2. Differentiate the surface area equation A=6x2A = 6x^2 with respect to time tt: dAdt=12xdxdt\frac{dA}{dt} = 12x \frac{dx}{dt}

Step 3: Solve for dxdt\frac{dx}{dt}

Using the equation from the volume: 6=3x2dxdt6 = 3x^2 \frac{dx}{dt} Solve for dxdt\frac{dx}{dt}: dxdt=63x2=2x2\frac{dx}{dt} = \frac{6}{3x^2} = \frac{2}{x^2}

Step 4: Substitute dxdt\frac{dx}{dt} into the Surface Area Equation

Now, substitute dxdt=2x2\frac{dx}{dt} = \frac{2}{x^2} into the equation for dAdt\frac{dA}{dt}: dAdt=12x×2x2=24x\frac{dA}{dt} = 12x \times \frac{2}{x^2} = \frac{24}{x}

So, the rate at which the surface area is increasing is: dAdt=24xcm2/s\frac{dA}{dt} = \frac{24}{x} \, \text{cm}^2/\text{s}

Final Answer:

The surface area is increasing at the rate of 24xcm2/s\frac{24}{x} \, \text{cm}^2/\text{s} when the edge length is xx cm.

Would you like to go over any step in more detail, or explore some variations of this problem?

Here are 5 related questions:

  1. What happens if the rate of change of the volume was different?
  2. How would the problem change if the shape were a sphere?
  3. What is the rate of change of the diagonal of the cube?
  4. How fast is the edge length increasing when the surface area is increasing at 10 cm²/s?
  5. How does the surface area change when the edge is doubled?

Tip: When solving related rates problems, make sure to differentiate each variable with respect to time carefully!

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Math Problem Analysis

Mathematical Concepts

Geometry
Related Rates
Differentiation

Formulas

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

Theorems

Related Rates Theorem
Differentiation with Respect to Time

Suitable Grade Level

Grades 11-12