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# Rates of change watch

1. So i have an exam in 8 hours time, and im still struggling to grasp rates of change.

Some of the questions im ok with, but some i just cannot pick out the info i need.

Im struggling with this one.

The volume of a spherical balloon is increasing at a constant rate of 0.25m^3s^-1
Find the rate at which the radius is increasing at an instant when the volume is 10m^3.

My working so far:

to find dr/dt so use the chain rule

dr/dt = dr/dv * dv/dt

dv/dt = 0.25
dv/dr = 4*pi*r^2 so dr/dv = 1/4*pi*r^2

So this is the point i get to. As i dont know the radius i dont know how to complete this, although im probably searching in the wrong place.
2. You want to know the radius at the instant v=10 (read question!)
Use formula for volume of sphere.

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Updated: April 6, 2011
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