A cylindrical tank of radius $10$ m is filled with wheat at $314$ m$^3$/h. The depth of the wheat is increasing at the rate of (take $\pi=3.14$)
A$0.1$ m/h
B$0.5$ m/h
C$1.1$ m/h
D$1$ m/h
Answer & Solution
Correct answer: D. $1$ m/h
1. Volume $V=\pi r^2 h=\pi(100)h=100\pi h$.
2. $\dfrac{dV}{dt}=100\pi\dfrac{dh}{dt}$.
3. Set $=314$: $314=100\pi\dfrac{dh}{dt}=314\dfrac{dh}{dt}$.
4. $\dfrac{dh}{dt}=\dfrac{314}{314}=1$ m/h.
5. Trap $0.1$ misplaces a power of ten.
_Source: syllabus Mathematics Ch 6 "Application of Derivatives", p.38_
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