Chapter 2.8, Problem 39E

Rate of change of the Arctic ice cap. In a trend that scientists attribute, at least in part, to global warming, the floating cap of sea ice on the Arctic Ocean has been shrinking since 1980. The ice cap always shrinks in summer and grows in winter. Average minimum size of the ice cap, in square miles, can be approximated by

$A=\pi r^2$.

In 2013, the radius of the ice cap was approximately 792 mi and was shrinking at a rate of approximately 4.7 mi/yr. (Source: Based on data from nsidc.org.) How fast was the area changing at that time?