A water tank is in the shape of a right circular cone as shown above. The diameter of the cone is 10 feet, and the height is 15 feet. The shape of the water in the tank is conical with radius r feet and height h feet. At noon, water is leaking from the bottom of the tank at a rate of 12 cubic feet per hour, and the volume of water in the tank is 27π cubic feet. At noon, what is the rate at which the height of the water in the tank is changing? (The volume V of a right circular cone with radius r and height h is V=13πr2h.)

Respuesta :

The rate at which the height of water tank changing is [tex]0.4244 \; \text {ft./hr}[/tex]

Step-by-step explanation:

Given information:

[tex]\text{\bold{The diameter of cone}}=10\;\rm{ ft.}[/tex]

[tex]\rm{The\; height\; of\; the\; cone}= 15\;\rm{ ft.}[/tex]

[tex]\text{The rate at which the water is leaking }= 12 \; \rm ft^3/hr.[/tex]

[tex]\text{Volume of water tank}=27 . \pi \; \rm ft^3[/tex]

Now,

If ,we take a right circular cone with radius r and height h , its volume

[tex]\text{volume (V)} = (1/3) \times \pi \times \rm{r^3 \times h[/tex]

Now, the radius of cone when the volume of water tank is given

[tex]\frac{1}{3}\times \pi \rm\times{r^2 \times h = 27 \times \pi[/tex]

The ratio of height to radius of the cone :

[tex]\text {h/r} =3[/tex]

[tex]\text r = \text h/3[/tex]

Now calculate :

[tex]dV/dt= dV/dh \times dh/dt[/tex]

Which gives:

[tex]12 = \frac{d(\text h^3/27 \pi)}{dh} \times \frac{d\text h}{dt} \\\\\frac{d\text h}{dt} = \frac{12}{\pi \text h^2/9}[/tex]

As ,

[tex]27=\frac{1}{27 \times h}[/tex]

[tex]\text h^3=27^2\\\text h= 9\\[/tex]

Now,

[tex]dh/dt=12/(\pi.\text h^2/9)\\dh/dt=4/3\pi\\dh/dt=0.4244\\[/tex]

Hence, The rate at which the height of water tank changing is [tex]0.4244 \; \text {ft./hr}[/tex]

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