Area of a circle (A) = πr2 Given dAdt=1.5cm2s−1 drdt = ? A = 2cm2 Now 2 = πr2 = r2=2π r = √2π cm = 0.798cm drdt=dAdt×drdt dAdr=2πr (differentiating A = πr2) drdA=12πr drdt=1.5×12×π×0.798=1.5×0.199 drdt=0.299cms−1 (to 3 s.f)
