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The radius of a cone is increasing at a rate of 3 centimeters per second and the height of the cone is decreasing at a rate of 4 centimeters per second.
At a certain instant, the radius is 8 centimeters and the height is 10 centimeters.
What is the rate of change of the volume of the cone at that instant (in cubic centimeters per second)?
Choose 1 answer:
(A) 
-(736 pi)/(3)
(B) 
-(224 pi)/(3)
(C) 
(736 pi)/(3)
(D) 
(224 pi)/(3)
The volume of 
a cone with radius 
r and height 
h is 
pir^(2)(h)/(3).

The radius of a cone is increasing at a rate of 33 centimeters per second and the height of the cone is decreasing at a rate of 44 centimeters per second.\newlineAt a certain instant, the radius is 88 centimeters and the height is 1010 centimeters.\newlineWhat is the rate of change of the volume of the cone at that instant (in cubic centimeters per second)?\newlineChoose 11 answer:\newline(A) 736π3 -\frac{736 \pi}{3} \newline(B) 224π3 -\frac{224 \pi}{3} \newline(C) 736π3 \frac{736 \pi}{3} \newline(D) 224π3 \frac{224 \pi}{3} \newlineThe volume of a a cone with radius r r and height h h is πr2h3 \pi r^{2} \frac{h}{3} .

Full solution

Q. The radius of a cone is increasing at a rate of 33 centimeters per second and the height of the cone is decreasing at a rate of 44 centimeters per second.\newlineAt a certain instant, the radius is 88 centimeters and the height is 1010 centimeters.\newlineWhat is the rate of change of the volume of the cone at that instant (in cubic centimeters per second)?\newlineChoose 11 answer:\newline(A) 736π3 -\frac{736 \pi}{3} \newline(B) 224π3 -\frac{224 \pi}{3} \newline(C) 736π3 \frac{736 \pi}{3} \newline(D) 224π3 \frac{224 \pi}{3} \newlineThe volume of a a cone with radius r r and height h h is πr2h3 \pi r^{2} \frac{h}{3} .
  1. Volume of Cone Formula: The formula for the volume of a cone is V=πr2h3V = \frac{\pi r^2 h}{3}. We need to find dVdt\frac{dV}{dt}, the rate of change of the volume.
  2. Chain Rule Application: To find dVdt\frac{dV}{dt}, we use the chain rule from calculus: dVdt=dVdrdrdt+dVdhdhdt\frac{dV}{dt} = \frac{dV}{dr} \cdot \frac{dr}{dt} + \frac{dV}{dh} \cdot \frac{dh}{dt}.
  3. Find dV/drdV/dr: First, we find dV/drdV/dr. Differentiate V=πr2h3V = \frac{\pi r^2 h}{3} with respect to rr, we get dV/dr=2πrh3dV/dr = \frac{2 \pi r h}{3}.
  4. Find dVdh\frac{dV}{dh}: Next, we find dVdh\frac{dV}{dh}. Differentiate V=πr2h3V = \frac{\pi r^2 h}{3} with respect to hh, we get dVdh=πr23\frac{dV}{dh} = \frac{\pi r^2}{3}.
  5. Substitute Values: Now we plug in the values for drdt\frac{dr}{dt}, dhdt\frac{dh}{dt}, rr, and hh. drdt=3cm/s\frac{dr}{dt} = 3 \, \text{cm/s}, dhdt=4cm/s\frac{dh}{dt} = -4 \, \text{cm/s}, r=8cmr = 8 \, \text{cm}, and h=10cmh = 10 \, \text{cm}.
  6. Calculate First Part: Substitute these values into the equation: dVdt=(2π8×103)×3+(π823)×(4)\frac{dV}{dt} = \left(\frac{2 \pi 8 \times 10}{3}\right) \times 3 + \left(\frac{\pi 8^2}{3}\right) \times (-4).
  7. Calculate Second Part: Calculate the first part: (2π810)/33=(2π80)/33=(160π)cm3/s(2 \cdot \pi \cdot 8 \cdot 10) / 3 \cdot 3 = (2 \cdot \pi \cdot 80) / 3 \cdot 3 = (160 \cdot \pi) \, \text{cm}^3/\text{s}.
  8. Add Parts Together: Calculate the second part: (π×82)/3×(4)=(π×64)/3×(4)=(256×π)cm3/s(\pi \times 8^2) / 3 \times (-4) = (\pi \times 64) / 3 \times (-4) = (-256 \times \pi) \, \text{cm}^3/\text{s}.
  9. Add Parts Together: Calculate the second part: (π82)/3(4)=(π64)/3(4)=(256π) cm3/s.(\pi \cdot 8^2) / 3 \cdot (-4) = (\pi \cdot 64) / 3 \cdot (-4) = (-256 \cdot \pi) \text{ cm}^3/\text{s}. Add the two parts together: dVdt=(160π)+(256π)=(96π) cm3/s.\frac{dV}{dt} = (160 \cdot \pi) + (-256 \cdot \pi) = (-96 \cdot \pi) \text{ cm}^3/\text{s}.

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