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Verified Answer :

dr/dt = -7 / (pir8)
 
         =  -7 / pi58
       
          = - 0.56 cm/min

Hope this helps



Edufirst

Surface area: S
radius: r
diameter: e
time: t

[dS/dt] = [dS/de][de/dt] => de/dt = [dS/dt] / [dS/de]

S = 4pir^2 = 4pi(e/2)^2 = pie^2 => dS/de =2pie

dS/dt = 7cm^2 / min

de / dt = [2pie] / [7cm^2/min] =[7cm^2/min] /  [2pi*10cm]  = 0.11 cm/min 




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