A cup has the shape of a right circular cone. The height of the cup is 12 cm, and the radius of the opening is 3 cm. Water is poured into the cup at a constant rate of 2 cm^3/sec. What is the rate at which the water level is rising when the depth of the water in the cup is 5 cm? (The volume of a cone of height h and radius r is given by V = 1/3 pi r^2 h)

(A) 32/25 pi cm/sec
(B) 96/125 pi cm/sec
(C) 2/3 pi cm/sec
(D) 2/9 pi cm/sec
(E) 1/200 pi cm/sec

Respuesta :

Answer:

(A) 32 / (25π) cm/sec

Step-by-step explanation:

Let's say h is the depth of the water and r is the radius of the water at that depth.

Using similar triangles, we can say:

r / h = 3 / 12

r / h = ¼

r = ¼ h

The volume of the water is:

V = ⅓ π r² h

Substituting:

V = ⅓ π (¼ h)² h

V = π h³ / 48

Take derivative with respect to time:

dV/dt = 3π h² / 48 dh/dt

dV/dt = π h² / 16 dh/dt

Solve for dh/dt:

dh/dt = 16 dV/dt / (π h²)

dV/dt = 2, and h = 5, so:

dh/dt = 16 (2) / (π 5²)

dh/dt = 32 / (25π)

The water rises at at rate of 32 / (25π) cm/sec.

Option A is correct. The rate at which the water level is rising when the depth of the water in the cup is 5 cm is [tex]\frac{32 \pi}{25}cm/sec[/tex]

The formula for calculating the volume of the right circular cone is expressed as [tex]V=\frac{1}{3}\pi r^2h[/tex] where:

  • r is the radius
  • h is the height of the cone

Using similar triangles:

[tex]\frac{h}{r}= \frac{12}{3}\\\frac{h}{r}= 4\\r = \frac{1}{4}h[/tex]

The volume of the expression becomes:

[tex]V=\frac{1}{3}\pi (\frac{1}{4} h)^2h\\V=\frac{1}{3} \pi \frac{h^3}{16}\\V=\frac{\pi h^3}{48}[/tex]

The formula for expressing the rate of change is volume is expressed as:

[tex]\frac{dV}{dt} = \frac{dV}{dh} \times \frac{dh}{dt}\\2 = \frac{dV}{dh} \times \frac{dh}{dt}\\2=\frac{3 \pi h^2}{48} \frac{dh}{dt}[/tex]

Given the following parameters

h = 5cm

Substitute:

[tex]2=\frac{3\pi (5)^2}{48}\frac{dh}{dt}\\2=\frac{75 \pi}{48} \frac{dh}{dt}\\96=75 \pi \frac{dh}{dt}\\ \frac{dh}{dt} = \frac{96\pi}{75}\\ \frac{dh}{dt} = \frac{32 \pi}{25} cm/sec[/tex]

Hence the rate at which the water level is rising when the depth of the water in the cup is 5 cm is [tex]\frac{32 \pi}{25}cm/sec[/tex]

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