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 When r = 5 cm, 
A
10
d
dr =
π Thus, the area of the circle is changing at the rate of
10π cm2/s Example 2 The volume of a cube is increasing at a rate of 9 cubic centimetres per
second