# math (Caluclas)

posted by .

A spherical ballon is leaking air at 2 cubic inches per hour.l How fast is the ballons radisu chancing when the radisu si 3 inches.

• math (Caluclas) -

v = 4/3 pi r^3

dv/dt = 4pi r^2 dr/dt

-2 = 4pi*9 dr/dt

dr/dt = -2/36pi = -1/18pi = -.0177 in/hr

• Caluclas ? -

Once I can understand as a typo, but twice ?

V= (4/3)πr^3
dV/dt = 4π r^2 dr/dt
-2 = 4π(9)dr/dt

etc.

• math (Caluclas) -

Sorry about that Reiny. I have a quick question though, where does the 9 come from?

• math (Caluclas) -

Notice both Steve and I have the same derivative equation of
dV/dt = 4π r^2 dr/dt

what is the value of r^2 when r = 3 ?

• math (Caluclas) -

Oh okay. I understand that, i was just not understand where the 9 came from, but i see now. Thank you both

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