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Exam question help!!

The height of tides in a harbour can be modelled by the equation H= a + bsin(30t) where H is the height in metres of the water t hours after midnight.
At 0300 the tide is 8m
At 0900 the tide is 2m
From this it can be found that a = 5 and b=3

Find for the first time, after midnight, when the height of the tide is 3m???

Tried this and got 30t = -41. 8
But has to be after so t must be positive. Help!
Reply 1
Avatar for John5678
John5678
1
Original post by Gemma_98
The height of tides in a harbour can be modelled by the equation H= a + bsin(30t) where H is the height in metres of the water t hours after midnight.
At 0300 the tide is 8m
At 0900 the tide is 2m
From this it can be found that a = 5 and b=3

Find for the first time, after midnight, when the height of the tide is 3m???

Tried this and got 30t = -41. 8
But has to be after so t must be positive. Help!


CCEA C2- recognise the question


According to the mark scheme, the answer can be the -41.8 or a positive 221.81

Other than that, I have no clue
(edited 7 years ago)
Reply 2
Avatar for Gemma_98
Gemma_98
OP
Original post by John5678
CCEA C2- recognise the question


According to the mark scheme, the answer can be the -41.8 or a positive 221.81

Other than that, I have no clue


Ino I get the -41.8 but where did the 221.81 come from? 😠
Reply 3
Avatar for B_9710
B_9710
18
There are infinitely many values of t for sin(30t)= -2/3. Also shouldn't you be in radians mode on calculator?
Original post by Gemma_98
Ino I get the -41.8 but where did the 221.81 come from? 😠


Cast diagram, 3rd quadrant since it's a negative.
Reply 5
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Gemma_98
OP
Original post by Middriver
Cast diagram, 3rd quadrant since it's a negative.


Ahh! Thank you 😄

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