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The radius of the base of a cone is decreasing at a rate of 2 centimeters per minute.
The height of the cone is fixed at 9 centimeters.
At a certain instant, the radius is 13 centimeters.
What is the rate of change of the volume of the cone at that instant (in cubic centimeters per minute)?
Choose 1 answer:
(A) 
-156 pi
(B) 
-78 pi
(C) 
-507 pi
(D) 
-12 pi
The volume of a cone with radius 
r and height 
h is 
pir^(2)(h)/(3).

The radius of the base of a cone is decreasing at a rate of 22 centimeters per minute.\newlineThe height of the cone is fixed at 99 centimeters.\newlineAt a certain instant, the radius is 1313 centimeters.\newlineWhat is the rate of change of the volume of the cone at that instant (in cubic centimeters per minute)?\newlineChoose 11 answer:\newline(A) 156π -156 \pi \newline(B) 78π -78 \pi \newline(C) 507π -507 \pi \newline(D) 12π -12 \pi \newlineThe volume of a cone with radius r r and height h h is πr2h3 \pi r^{2} \frac{h}{3} .

Full solution

Q. The radius of the base of a cone is decreasing at a rate of 22 centimeters per minute.\newlineThe height of the cone is fixed at 99 centimeters.\newlineAt a certain instant, the radius is 1313 centimeters.\newlineWhat is the rate of change of the volume of the cone at that instant (in cubic centimeters per minute)?\newlineChoose 11 answer:\newline(A) 156π -156 \pi \newline(B) 78π -78 \pi \newline(C) 507π -507 \pi \newline(D) 12π -12 \pi \newlineThe volume of a cone with radius r r and height h h is πr2h3 \pi r^{2} \frac{h}{3} .
  1. Volume Formula Derivation: The formula for the volume of a cone is V=13πr2hV = \frac{1}{3} \pi r^2 h. We need to find the derivative of the volume with respect to time, dVdt\frac{dV}{dt}.
  2. Constant Height Consideration: Given that the height hh is constant at 99 cm, we can treat it as a constant when differentiating.
  3. Differentiation with Respect to Time: Differentiate the volume formula with respect to time to get dVdt=(13)πh2rdrdt\frac{dV}{dt} = \left(\frac{1}{3}\right) \cdot \pi \cdot h \cdot 2r \cdot \frac{dr}{dt}, since d(r2)dt=2rdrdt\frac{d(r^2)}{dt} = 2r \cdot \frac{dr}{dt}.
  4. Substitution of Values: Plug in the values: h=9cmh = 9 \, \text{cm}, r=13cmr = 13 \, \text{cm}, and drdt=2cm/min\frac{dr}{dt} = -2 \, \text{cm/min} (since the radius is decreasing).
  5. Calculation of dVdt\frac{dV}{dt}: Calculate dVdt=(13)π9213(2)\frac{dV}{dt} = \left(\frac{1}{3}\right) \cdot \pi \cdot 9 \cdot 2 \cdot 13 \cdot (-2).
  6. Final Simplification: Simplify the expression: dVdt=(13)π9213(2)=156π\frac{dV}{dt} = \left(\frac{1}{3}\right) * \pi * 9 * 2 * 13 * (-2) = -156 \pi cubic centimeters per minute.

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