Posted by **jack** on Thursday, August 11, 2011 at 6:33pm.

A Glass Of Water Is Being Filled With Water At The Rate Of 12 Pi Cm^3/sec

And the radius of the top is 6 cm and the radius of the base is 3 cm and the height of the glass is 12 cm , find A) dh/dt and B) radius when the height of water is half?

- math -
**bobpursley**, Thursday, August 11, 2011 at 7:39pm
well, this is a truncated cone. Volume of the full cone is 1/3 area base*height. So would be total height if it were not truncated:

By proportions: heighttotal/areatop=(heighttotal-12)/areabottom

height total=6(heighttotal-12)/3

h=2h-48

h=48,

but r is a function height.

r=3+h/4

dr=dh/4

now looking at incremental volumes

dVolume=PI*r^2*dh where r=3+h/4 or

dvolume=PI (3+h/4)^2 dh

change variables, let u= 3+h/4

du= dh/4 when h=0 u=3, when h=h, u=3+h/4

volume= INT(PI/4) u^2 du over limits

volume= PI/12 [(3+h/4)^3-3^3] check that.

a) dv/dt=PI/4 (3+h/4)^2 dh/dt

h=6, find dh/dt given dv/dt=12PI

b) r when h=6; too easy, we did that already. r=3+6/4

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