Let $V$ be the volume of a cylinder having height $h$ and radius $r$, and assume that $h$ and $r$ vary with time.
(a) How are $d V / d t, d h / d t$, and $d r / d t$ related?
(b) At a certain instant, the height is 6 in and increasing at $1 \mathrm{in} / \mathrm{s}$, while the radius is 10 in and decreasing at $1 \mathrm{in} / \mathrm{s}$. How fast is the volume changing at that instant? Is the volume increasing or decreasing at that instant?