Date | November 2021 | Marks available | 2 | Reference code | 21N.2.SL.TZ0.8 |
Level | Standard Level | Paper | Paper 2 | Time zone | Time zone 0 |
Command term | Find | Question number | 8 | Adapted from | N/A |
Question
The height of water, in metres, in Dungeness harbour is modelled by the function H(t)=a sin(b(t-c))+d, where t is the number of hours after midnight, and a, b, c and d are constants, where a>0, b>0 and c>0.
The following graph shows the height of the water for 13 hours, starting at midnight.
The first high tide occurs at 04:30 and the next high tide occurs 12 hours later. Throughout the day, the height of the water fluctuates between 2.2 m and 6.8 m.
All heights are given correct to one decimal place.
Show that b=π6.
Find the value of a.
Find the value of d.
Find the smallest possible value of c.
Find the height of the water at 12:00.
Determine the number of hours, over a 24-hour period, for which the tide is higher than 5 metres.
Markscheme
12=2πb OR b=2π12 A1
b=π6 AG
[1 mark]
a=6.8-2.22 OR a=max-min2 (M1)
=2.3 (m) A1
[2 marks]
d=6.8+2.22 OR d=max+min2 (M1)
=4.5 (m) A1
[2 marks]
METHOD 1
substituting t=4.5 and H=6.8 for example into their equation for H (A1)
6.8=2.3 sin(π6(4.5-c))+4.5
attempt to solve their equation (M1)
c=1.5 A1
METHOD 2
using horizontal translation of 124 (M1)
4.5-c=3 (A1)
c=1.5 A1
METHOD 3
H'(t)=(2.3)(π6)cos(π6(t-c)) (A1)
attempts to solve their H'(4.5)=0 for c (M1)
(2.3)(π6)cos(π6(4.5-c))=0
c=1.5 A1
[3 marks]
attempt to find H when t=12 or t=0, graphically or algebraically (M1)
H=2.87365…
H=2.87 (m) A1
[2 marks]
attempt to solve 5=2.3 sin(π6(t-1.5))+4.5 (M1)
times are t=1.91852… and t=7.08147… , (t=13.9185…, t=19.0814…) (A1)
total time is 2×(7.081…-1.919…)
10.3258…
=10.3 (hours) A1
Note: Accept 10.
[3 marks]