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The surface area of a cube is increasing at a rate of 15 square meters per hour.
At a certain instant, the surface area is 24 square meters.
What is the rate of change of the volume of the cube at that instant (in cubic meters per hour)?
Choose 1 answer:
(A) 8
(B) 
(15)/(2)
(C) 
(sqrt15)^(3)
(D) 
(5)/(8)

The surface area of a cube is increasing at a rate of 1515 square meters per hour.\newlineAt a certain instant, the surface area is 2424 square meters.\newlineWhat is the rate of change of the volume of the cube at that instant (in cubic meters per hour)?\newlineChoose 11 answer:\newline(A) 88\newline(B) 152 \frac{15}{2} \newline(C) (15)3 (\sqrt{15})^{3} \newline(D) 58 \frac{5}{8}

Full solution

Q. The surface area of a cube is increasing at a rate of 1515 square meters per hour.\newlineAt a certain instant, the surface area is 2424 square meters.\newlineWhat is the rate of change of the volume of the cube at that instant (in cubic meters per hour)?\newlineChoose 11 answer:\newline(A) 88\newline(B) 152 \frac{15}{2} \newline(C) (15)3 (\sqrt{15})^{3} \newline(D) 58 \frac{5}{8}
  1. Surface Area Calculation: The surface area of a cube is 66 times one of its side's square, so one side's square is 24m26\frac{24 \, \text{m}^2}{6}.\newlineCalculate the length of one side.\newlineSide length = 24m26=4m2=2m\sqrt{\frac{24 \, \text{m}^2}{6}} = \sqrt{4 \, \text{m}^2} = 2 \, \text{m}.
  2. Volume Calculation: The volume of a cube is side length cubed, V=s3V = s^3. Calculate the volume with the side length. Volume = 2m×2m×2m=8m32 \, \text{m} \times 2 \, \text{m} \times 2 \, \text{m} = 8 \, \text{m}^3.
  3. Surface Area Rate of Change: The surface area is increasing at a rate of 15m2/hour15 \, \text{m}^2/\text{hour}. Since the surface area is 6×side length26 \times \text{side length}^2, d(SA)dt=6×2×side length×d(side length)dt\frac{d(SA)}{dt} = 6 \times 2 \times \text{side length} \times \frac{d(\text{side length})}{dt}.
  4. Side Length Rate of Change: Solve for d(side length)dt\frac{d(\text{side length})}{dt}.15m2/hour=6×2×2m×d(side length)dt15 \, \text{m}^2/\text{hour} = 6 \times 2 \times 2 \, \text{m} \times \frac{d(\text{side length})}{dt}.d(side length)dt=15m2/hour(6×2×2m)=15m2/hour24m=58m/hour\frac{d(\text{side length})}{dt} = \frac{15 \, \text{m}^2/\text{hour}}{(6 \times 2 \times 2 \, \text{m})} = \frac{15 \, \text{m}^2/\text{hour}}{24 \, \text{m}} = \frac{5}{8} \, \text{m/hour}.
  5. Volume Rate of Change: The rate of change of the volume is given by dVdt=3×side length2×d(side length)dt\frac{dV}{dt} = 3 \times \text{side length}^2 \times \frac{d(\text{side length})}{dt}. Calculate dVdt\frac{dV}{dt} using the side length and d(side length)dt\frac{d(\text{side length})}{dt}. dVdt=3×(2m)2×(58m/hour)=3×4m2×58m/hour=152m3/hour.\frac{dV}{dt} = 3 \times (2 \, \text{m})^2 \times \left(\frac{5}{8} \, \text{m/hour}\right) = 3 \times 4 \, \text{m}^2 \times \frac{5}{8} \, \text{m/hour} = \frac{15}{2} \, \text{m}^3/\text{hour}.

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