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Math
Posted 11 months ago
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Water drips out of a hole at the vertex of an upside down cone at a rate of 3 cm^3 per minute. The cone’s height and radius are 2 cm and 1 cm, respectively. At what rate does the height of the water change when the water level is half a centimeter below the top of the cone? The volume of a cone is V = (π/3)*r^2*h, where r is the radius and h is the height of the cone.
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Answer from Sia
Posted 11 months ago
Solution by Steps
step 1
Given the volume of a cone V=π3r2h V = \frac{\pi}{3} r^2 h , differentiate both sides with respect to time t t to find the related rates
step 2
Applying the chain rule, we get dVdt=π3(2rhdrdt+r2dhdt) \frac{dV}{dt} = \frac{\pi}{3} (2rh \frac{dr}{dt} + r^2 \frac{dh}{dt})
step 3
Given dVdt=3 \frac{dV}{dt} = -3 cm³/min (since water is dripping out), h=1.5 h = 1.5 cm, and r=h2 r = \frac{h}{2} , we substitute these values into the differentiated equation
step 4
Substituting r=h2 r = \frac{h}{2} into the equation, we get dVdt=π3(2h2hdrdt+(h2)2dhdt) \frac{dV}{dt} = \frac{\pi}{3} (2 \cdot \frac{h}{2} \cdot h \cdot \frac{dr}{dt} + (\frac{h}{2})^2 \cdot \frac{dh}{dt})
step 5
Since r r is a function of h h , drdt \frac{dr}{dt} can be expressed in terms of dhdt \frac{dh}{dt} . Differentiating r=h2 r = \frac{h}{2} with respect to t t , we get drdt=12dhdt \frac{dr}{dt} = \frac{1}{2} \frac{dh}{dt}
step 6
Substituting drdt \frac{dr}{dt} into the equation from step 4, we have 3=π3(2h2h12dhdt+(h2)2dhdt) -3 = \frac{\pi}{3} (2 \cdot \frac{h}{2} \cdot h \cdot \frac{1}{2} \frac{dh}{dt} + (\frac{h}{2})^2 \cdot \frac{dh}{dt})
step 7
Simplify the equation to solve for dhdt \frac{dh}{dt} when h=1.5 h = 1.5 cm
step 8
After simplification, we get 3=π3(34h2dhdt) -3 = \frac{\pi}{3} (\frac{3}{4} h^2 \frac{dh}{dt})
step 9
Substitute h=1.5 h = 1.5 cm into the equation to find dhdt \frac{dh}{dt}
step 10
Solving for dhdt \frac{dh}{dt} , we get dhdt=3π3341.52 \frac{dh}{dt} = \frac{-3}{\frac{\pi}{3} \cdot \frac{3}{4} \cdot 1.5^2}
step 11
Calculate the value of dhdt \frac{dh}{dt} to find the rate at which the height of the water changes
step 12
The final value of dhdt \frac{dh}{dt} is dhdt=3π3341.52=3π42.25=39π4=349π=129π=43π \frac{dh}{dt} = \frac{-3}{\frac{\pi}{3} \cdot \frac{3}{4} \cdot 1.5^2} = \frac{-3}{\frac{\pi}{4} \cdot 2.25} = \frac{-3}{\frac{9\pi}{4}} = \frac{-3 \cdot 4}{9\pi} = \frac{-12}{9\pi} = \frac{-4}{3\pi} cm/min
Answer
dhdt=43π \frac{dh}{dt} = \frac{-4}{3\pi} cm/min
Key Concept
Related Rates in Conical Volume Change
Explanation
The rate of change of the height of the water in the cone is found by differentiating the volume formula with respect to time and substituting the given values to solve for dhdt \frac{dh}{dt} .

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