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The radius of a disk grows at 2cm/s. Calculate the growth rate of the disks area when the radius mesures 7 cm. (Designate the area of the disk with A and the radius with r)

A) Find the relation between A and r

B) Find the derivative of this relation on time t
dA/dt =

C) When r = 7cm and dr/dt = 2 cm/s, find dA/dt

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I'm just not sure for A, is the relation going to be dA/dt = dr/dA * dr/dt??

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