Maximum depths of the river

Main Question or Discussion Point

D = 9 + 3cos[pi(t)/14]
where D meters is the depth of the water above the river bed at time t hours after 12 noon on 7 March 2005, Monday. This means that the depth of water only depends on time.
a) Write down the minimum and maximum depths of the river
b) Find the day and time when the water first reaches its minimum.

VietDao29
Homework Helper
You know that:
$$-1 \leq \cos \alpha \leq 1$$
So when the D is minimum is when $$\cos \frac{\pi t}{14}$$ is minimum and vice versa, when the D is maximum is when $$\cos \frac{\pi t}{14}$$ is maximum.
Can you go from here?
Viet Dao,

Yup. i got there. So minimum is 6m, maximum is 12m. But i dont know how to manipulate it formula to solve the second part.

EnumaElish