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Problem 20 - Entrance Test

A spherical balloon rises from an Algerian desert such that its radius increases at 1 cm⁄s. When radius is 10 cm, the rate of increase of its surface area (cm²⁄s) is

Correct: C

A=4πr² ⇒ dA⁄dt=8πr dr⁄dt=8π·10·1=80π.