Let us consider some examples Example 1 Find the rate of change of the area of a circle per second with respect to
its radius r when r = 5 cm Solution The area A of a circle with radius r is given by A = πr2 Therefore, the rate
of change of the area A with respect to its radius r is given by 
2
A
(
)
2
d
d
r
r
dr
=dr
π
= π