rage tank needs to create a tank with a radius of 3 meters.
    Recall that the volume of a sphere is $V(r) = \frac{4}{3}\pi
    r^3$. No manufacturing process is perfect, so the resulting
    sphere will have a radius of 3 meters, plus or minus some small
    amount $dr$. The actual radius will be $3+dr$. Find the
    differential $dV$. Then use differentials to estimate the
    change in the volume of the sphere if the actual radius is 3.02
    meters instead of the planned 3 meters.
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