Hi Sonam, the area A(r) of a circle of radius r is given by: A(r) = π*r^{2} Thus, using the chain rule, dA/dt = dA/dr * dr/dt = 2πr*dr/dt. The answer to part (b) falls out when you substitute r = 37m and dr/dt = 1 m/s.

Note that the formula for dA/dt includes the product of circumference and the differential width dr: 2πr*dr. This makes sense geometrically as this product represents the differential area during expansion.

Robert