Posted by **Katie** on Sunday, February 6, 2011 at 7:14pm.

If water is poured into a cup at a rate of 1 cubic centimeter per second, how fast is the dept of the water increasing when the water is 4 cm deep?

r = 6 cm

h = 9 cm

V = 1/3(pie) r^2 x h

V = (pie)r'h + 1/3 (pie)r^2 h'

6 / 9 = r / h

6h = 9r

r = 3 / 4.5 h

V = 1/3(pie) (3/4.5h)^2

My answer came out to 9 / 64(pie) cm/sec.

- Calculus -
**bobpursley**, Sunday, February 6, 2011 at 7:33pm
what is the shape of the container?

- Calculus -
**Katie**, Sunday, February 6, 2011 at 8:08pm
The shape is a cone. The radius is 6 cm and the height is 9 cm

- Calculus -
**Reiny**, Sunday, February 6, 2011 at 8:40pm
Your answer is correct if you write it as

9/(64pi).

I am curious why you went from

6h = 9r to

r = 3/4.5

why not just do

r = 6h/9 = 2h/3

instead of having decimals in your fraction.

Either we use fractions or we use decimals, but we generally don't mix them together.

- Calculus -
**helper**, Sunday, February 6, 2011 at 9:52pm
I think you have a mistake. I was working on this problem at the same time.

V = 1/3 pi r^2 h

Only 'r' is squared. It appears that you squared 'h' too.

V = 1/3(pie) (3/4.5h)^2

Anyway, even with squaring h too, I don't come up with 9/64 pi.

- Helper - Calculus -
**Reiny**, Sunday, February 6, 2011 at 10:03pm
Here is what I did

from above

r = 2h/3

V = (1/3)π(r^2)h

= (1/3)π(4h^2/9)h

= (4π/27) h^3

dV/dt = (4π/9)h^2 dh/dt

when dV/dt = 1 and h = 4

1 = (4π/9)(16)dh/dt

dh/dt = 9/(64π)

(notice that Katie had r = 3h/4.5 which I found strange since that is simply 2h/3 )

- Calculus -
**helper**, Sunday, February 6, 2011 at 10:59pm
I used r = 2/3 instead of r = 2/3 h!

Thanks so much REINY.

I really appreciate you taking the time to explain this to me.

I am teaching myself calculus, and sometimes the book just doesn't explain things well.

The r = 3h/4.5 threw me at first, until I figured out what she did (I had 2h/3).

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