Question

the volume of an expanding sphere is increasing at a rate of 12 cubic feet per second. when the volume of the sphere is 36 pi cubic feet, how fast, in square feet per second, is the surface area increasing?

Answer 1

Surface are increasing at the rate of 8 square feet per second.

What is a sphere?

A sphere is a geometrical object that resembles a two-dimensional circle in three dimensions. In three-dimensional space, a sphere is a collection of points that are all located at the same r-distance from a single point. The radius of the sphere is equal to r, and the provided point is its centre.

Volume(V) = $4\pi r^3$

            $\frac{dV}{dt} =\frac{4\pi }{3} \times3r^2\times\frac{dr}{dt} =4\pi r^2.\frac{dr}{dt} \\\\\boxed{\frac{dr}{dt} =\frac{1}{4\pi r^2} (\frac{dV}{dt} )}$

             $V=36\pi \\\\\frac{4}{3} \pi r^3=36\pi \\\\r^3=\frac{108}{4} =27\\\\\boxed{r=3}\\\\\boxed{\frac{dV}{dt} =12}$

Surface area(S) = $4\pi r^2$

                        $\frac{dS}{dr} =8\pi r\times\frac{dr}{dt} \\\\\frac{dS}{dt} =\frac{8\pi r}{4\pi r^2} \times\frac{dr}{dt}\\\\ \frac{dS}{dt}=\frac{2}{r} \times \frac{dV}{dt} =\frac{2}{3} \times12=8\\\\\boxed{\frac{dS}{dt} =8}$

Hence, the surface area increasing at the rate of 8 square feet per second.

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