The radius of a blot ink is increasing at a rate of 1.55mm per seconds.find the rate at which the area is increasing after 4 seconds

Answer 1

a = pi r^2
da/dt = 2pi r dr/dt

If the ink blot started with radius of zero, then after 4 seconds it will have a radius of 4*1.55 = 6.2mm

da/dt = 2pi * 6.2 * 1.55 = 60.38 mm^2/sec