안녕하세요 선생님, 항상 감사드립니다.

제가 2019년에 제출된 frq 문제를 풀고 있는데요, 4번째 문제의 b번 답이 이해가 안됩니다. 문제에서 이미 rate of change of height with respect to time을 이미 주었는데, 그 식에 왜 derivative를 한번 더 해야 하는 건가요?

문제는 다음과 같습니다:

A cylindrical barrel with a diameter of 2 feet contains collected rainwater, as shown in the figure above. The

water drains out through a valve (not shown) at the bottom of the barrel. The rate of change of the height h of

the water in the barrel with respect to time t is modeled by dh/dt = -(1/10)h^(1/2), where h is measured in feet and 

t is measured in seconds. (The volume V of a cylinder with radius r and height h is V = (pi)r^2h.)

(b) When the height of the water is 3 feet, is the rate of change of the height of the water with respect to time increasing or decreasing? Explain your reasoning. 

감사합니다.