Posted by **Erica** on Tuesday, October 3, 2006 at 12:18pm.

The surface ares S (in square meters) of a hot air ballon is given by s(r)=4 pi r^2 where r is radius of balloon (in meters). If the radius r is increasing with time (in seconds) according to the formula r(t) = 2/3t^3, t is greater than or equal to 0, find the surface area S of the balloon as a function of the time t.

Ok, your first function give S(r) or surface area as a function of the radius. You second function r(t)=(2/3)t^3 gives the radius as a function of time. If you compose the function to get

S(r(t)) = 4*pi*((2/3)t^3)^2

you have the surface area as a function of time alone. You can write it as

S(t) = something you can simplify.

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