- Difficulty
- Standard
- Suggested time
- 4 min
- Marks
- 2
Power functions and first sketches practice question 12
A person’s core body temperature, in degrees Celsius, is modelled by the function below, where is the time in hours after midnight. At what time , with , is the temperature highest?
Only one part of changes with . When is that part as large as it can be?
The constant and the positive amplitude do not move the peak. is largest when the sine equals , which happens when its input is plus a whole number of turns.
Solve , and check that your lies between and .
A number of hours after midnight, for example 9.5.
Theory UBC Math 100 §1.5Key results
Worked solution
Strategy
A sinusoid A + B sin(…) with B > 0 is highest exactly when the sine equals 1. Set the input of the sine equal to π/2 plus whole turns, solve for t, and keep the solution inside the day.
Working
The period is 24 hours: one full cycle per day, so there is one peak in any 24-hour window.
Sine reaches its maximum of 1 at π/2 and at every full turn after or before it.
Multiply through by 12/π.
Only k = 0 lands inside the day: the peak is at 5 p.m., at 37.3 °C.
Answer
The answer is 17 hours after midnight, that is 5 p.m..
Check
T(16) and T(18) are both about 37.29 °C, equal and just below T(17) = 37.3 °C, so the peak sits at 17.
Where this goes wrong
Moving the peak the wrong way and answering 19 (that is, −5 + 24). Inside the sine, t − 11 delays the cycle by 11 hours, so the peak of sin(πt/12) at t = 6 moves later, to t = 17.
Try a similar question
Find the sum of all solutions of the equation below in the interval .Saved in this browser only.