(a) If A is the area of a circle with radius r and the circle expands as time passes, find dA/dt
in terms of dr/dt.
dA/dt = (2\pi r)dr/dt
(b) Suppose oil spills from a ruptured tanker and spreads in a circular pattern. If the radius of the oil spill increases at a constant rate of 2 m/s, exactly how fast (in m^2/s) is the area of the spill increasing when the radius is
35 m?