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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) 
-78 pi
(B) 
-156 pi
(C) 
-12 pi
(D) 
-507 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) 78π -78 \pi \newline(B) 156π -156 \pi \newline(C) 12π -12 \pi \newline(D) 507π -507 \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) 78π -78 \pi \newline(B) 156π -156 \pi \newline(C) 12π -12 \pi \newline(D) 507π -507 \pi \newlineThe volume of a cone with radius r r and height h h is πr2h3 \pi r^{2} \frac{h}{3} .
  1. Given Information: Given: \newlineRate of change of radius drdt=2\frac{dr}{dt} = -2 cm/min (since the radius is decreasing)\newlineHeight of cone h=9h = 9 cm (constant)\newlineRadius r=13r = 13 cm\newlineVolume of a cone V=13πr2hV = \frac{1}{3}\pi r^2 h\newlineWe need to find dVdt\frac{dV}{dt}.
  2. Differentiate Volume Formula: Differentiate the volume formula with respect to time tt to find dVdt\frac{dV}{dt}.
    dVdt=13π(2rhdrdt+r2dhdt)\frac{dV}{dt} = \frac{1}{3}\pi(2rh \frac{dr}{dt} + r^2 \frac{dh}{dt})
    Since the height is constant, dhdt=0\frac{dh}{dt} = 0, so the equation simplifies to:
    dVdt=13π(2rhdrdt)\frac{dV}{dt} = \frac{1}{3}\pi(2rh \frac{dr}{dt})
  3. Substitute Given Values: Substitute the given values into the differentiated equation.\newlinedVdt=13π(2×9×13×(2))\frac{dV}{dt} = \frac{1}{3}\pi(2 \times 9 \times 13 \times (-2))\newlinedVdt=13π(2×9×13×(2))=234π\frac{dV}{dt} = \frac{1}{3}\pi(2 \times 9 \times 13 \times (-2)) = -234\pi

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