The area of a circle (A) with radius (r) is given by,
A=πr2
Now, the rate of change of the area with respect to its radius is given by,
\\(\\frac{dA}{dr}\\) = \\(\\frac{d}{dr}\\) (πr2)=2πr
At r=3 cm,
\\(\\frac{dA}{dr}\\) =2π(3) = 6π
At r=4 cm,
\\(\\frac{dA}{dr}\\) = 2π(4) =8π