The volume of a sphere increases at the rate of 2 cm 3 per second. Find the rate at which the surface area increases when the radius is 3 cm.
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Volume of sphere = 3 4 π r 3 d t d V = 4 π r 2 ⋅ d t d r ⟹ 2 = 4 π r 2 d t d r . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ( ∵ d t d V is given ) ⟹ d t d r = 2 π r 2 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ( 1 ) Now, Surface area = 4 π r 2 ⟹ d t d S = 8 π r ⋅ d t d r ⟹ d t d S = 2 π r 2 8 π r . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ( b y ( 1 ) ) ⟹ d t d S = r 4 d t d S r = 3 c m = 3 4 c m 2 / s e c
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V = 3 4 π r 3
d t d V = 4 π r 2 d t d r
V ′ ( 3 ) = 4 π × 9 d t d r = 2
d t d r = 1 8 π 1
S = 4 π r 2
d t d S = 8 π r d t d r
S ′ ( 3 ) = 8 π × 3 × 1 8 π 1 = 3 4