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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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