differential calculus
posted by wen .
Find how fast (a) the circumference (b)the area, if a cicle increases when the radius increases.

For absolute change rates:
dC/dt = 2*pi*dR/dt
dA/dt = 2*pi*R*dR/dt
For relative change rates
(1/C)dC/dt = (1/R) dR/dt
(1/A) dA/dt = (2/R)*dR/dt