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Find the rate of change of the area of a circle with respect to its radius r when r = 4 cm

1. b) Find the rate of change of the area of a circle with respect to its radius r when
 r = 4 cm

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Area of the circle (A) =  \pi r^{2}
Rate of change of the area of a circle with respect to its radius r = \frac{dA}{dr} = \frac{d(\pi r^{2})}{dr} = 2 \pi r
So, when r = 4, Rate of change of the area of a circle = 2 \pi (4)  = 8 \pi
Hence, Rate of change of the area of a circle with respect to its radius r when r = 4 is  8 \pi

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