2016 ONE(b)

Achievement
Question
The height of the tide at a particular beach today is given by the function

h(t)=0.8sin(4π25t+π2)h(t) = 0.8\sin\left(\frac{4\pi}{25}t+\frac{\pi}{2}\right)

where hh is the height of water, in metres, relative to the mean sea level
and tt is the time in hours after midnight.
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At what rate was the height of the tide changing at that beach at 9.00 a.m. today?
Official Answer
dhdt=3.2π25cos(4π25t+π2)\dfrac{dh}{dt}=\dfrac{3.2\pi}{25}\cos\left(\dfrac{4\pi}{25}t+\dfrac{\pi}{2}\right)
=0.402cos(36π25+π2)=0.402\cos\left(\dfrac{36\pi}{25}+\dfrac{\pi}{2}\right)
=0.395=0.395 metres per hour
Grading Criteria

Achievement (u)

  • Correct solution with correct derivative

Merit (r)

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Excellence T1

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Excellence T2

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