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How fast the area of a circle increases ...

How fast the area of a circle increases when its radius is 5cm.
(i) respect to radius (ii) with respect to diameter.

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(i)`A =pir^(2) ," "(dA)/(dr) =2pir`
`:. (dA)/(dr)]_(r=5) " " =10 pi cm^(2)//cm.`
`(ii) A =(pi)/(4) D^(2) , (dA)/(dD) =(pi)/(2)D`
`:. (dA)/(dD)]_(D=10)=(pi)/(2). 10=5pi cm^(2)//cm.`
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