help please :) During a fishing trip Alex notices that the height h of the tide (in metres) is given by h=2 + 0.5cos(pi/4*t) where t is measured in hours from the start of the trip.
Find the rate of change of the height 2 hours after the start of the trip.

Question

help please :) During a fishing trip Alex notices that the height h of the tide (in metres) is given by h=2 + 0.5cos(pi/4*t) where t is measured in hours from the start of the trip. 
Find the rate of change of the height 2 hours after the start of the trip.

butterflydreamer · Answer

do you just derive h and then sub in t = 2??

butterflydreamer · Answer

if you derive $$h = 2 + 0.5 \cos (\frac{ \pi }{ 4 } t)$$
you get :
$$h' = -\frac{\pi}{8} \sin( \frac{ \pi }{ 4 }t)$$

kky10997 · Answer

now but t =2hr

butterflydreamer · Answer

so.. $$h' = \frac{ -\pi }{ 8 } \sin(\frac{ \pi }{ 2 }) = -\frac{ \pi }{ 8 }$$ ?

kky10997 · Answer

yeah

butterflydreamer · Answer

oh thank you (:

kky10997 · Answer

:)

butterflydreamer · Answer

i just checked and it says that it is incorrect :/

rishavraj · Answer

is it
$$-0.5\frac{ \pi }{ 4 }$$

butterflydreamer · Answer

nope

rishavraj · Answer

hold on wht is it then ??

rishavraj · Answer

$$-\frac{ \pi }{ 8 }$$

butterflydreamer · Answer

.... no. -0.5(pi/4) is the same as -pi/8

rishavraj · Answer

lol yeah bt whts the answer given??

butterflydreamer · Answer

i don't know what it is LOL but it comes up as being incorrect :/

rishavraj · Answer

hmmmm @butterflydreamer  i guess the differentiation is correct though

butterflydreamer · Answer

yeah. it's strange... not sure why the answer is wrong.. unless we're interpreting the question wrong