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The radius of the base of a cylinder is increasing at a rate of 1 meter per hour and the height of the cylinder is decreasing at a rate of 4 meters per hour.
At a certain instant, the base radius is 5 meters and the height is 8 meters.
What is the rate of change of the volume of the cylinder at that instant (in cubic meters per hour)?
Choose 1 answer:
(A) 
-20 pi
(B) 
-180 pi
(C) 
20 pi
(D) 
180 pi
The volume of a cylinder with base radius 
r and height 
h is 
pir^(2)h.

The radius of the base of a cylinder is increasing at a rate of 11 meter per hour and the height of the cylinder is decreasing at a rate of 44 meters per hour.\newlineAt a certain instant, the base radius is 55 meters and the height is 88 meters.\newlineWhat is the rate of change of the volume of the cylinder at that instant (in cubic meters per hour)?\newlineChoose 11 answer:\newline(A) 20π -20 \pi \newline(B) 180π -180 \pi \newline(C) 20π 20 \pi \newline(D) 180π 180 \pi \newlineThe volume of a cylinder with base radius r r and height h h is πr2h \pi r^{2} h .

Full solution

Q. The radius of the base of a cylinder is increasing at a rate of 11 meter per hour and the height of the cylinder is decreasing at a rate of 44 meters per hour.\newlineAt a certain instant, the base radius is 55 meters and the height is 88 meters.\newlineWhat is the rate of change of the volume of the cylinder at that instant (in cubic meters per hour)?\newlineChoose 11 answer:\newline(A) 20π -20 \pi \newline(B) 180π -180 \pi \newline(C) 20π 20 \pi \newline(D) 180π 180 \pi \newlineThe volume of a cylinder with base radius r r and height h h is πr2h \pi r^{2} h .
  1. Volume Formula: The formula for the volume of a cylinder is V=πr2hV = \pi r^2 h. We need to find the rate of change of the volume, which is dVdt\frac{dV}{dt}.
  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. Calculate dV/drdV/dr: First, we find dV/drdV/dr which is the derivative of VV with respect to rr. dV/dr=2πrhdV/dr = 2\pi rh.
  4. Calculate dVdh\frac{dV}{dh}: Then we find dVdh\frac{dV}{dh} which is the derivative of VV with respect to hh. dVdh=πr2\frac{dV}{dh} = \pi r^2.
  5. Find drdt\frac{dr}{dt} and dhdt\frac{dh}{dt}: Now we plug in the values for drdt\frac{dr}{dt} and dhdt\frac{dh}{dt}. drdt\frac{dr}{dt} is 11 meter per hour and dhdt\frac{dh}{dt} is 4-4 meters per hour.
  6. Plug in Values: We also plug in the values for rr and hh at the instant we are considering. rr is 55 meters and hh is 88 meters.
  7. Calculate dVdt\frac{dV}{dt}: Now we calculate dVdt\frac{dV}{dt} using the values we have. dVdt=2π(5 meters)(8 meters)(1 meter/hour)+π(5 meters)2(4 meters/hour)\frac{dV}{dt} = 2\pi(5 \text{ meters})(8 \text{ meters})(1 \text{ meter/hour}) + \pi(5 \text{ meters})^2(-4 \text{ meters/hour}).
  8. Simplify Expression: Simplify the expression. dVdt=80π\frac{dV}{dt} = 80\pi meters3^3/hour + (100π)(-100\pi) meters3^3/hour.
  9. Combine Terms: Combine the terms to find the total rate of change of the volume. dVdt=80π100π\frac{dV}{dt} = 80\pi - 100\pi meters3^3/hour.
  10. Final Result: Simplify the result. dVdt=20π\frac{dV}{dt} = -20\pi meters3^3/hour.

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