The cosine function with the given characteristics is:
f(x) = 3*cos(x/2 - pi).
How to get the cosine function?
The general cosine function is:
f(x) = A*cos(kx + p)
Where A is the amplitude and p is the phase, then we know that:
A = 3
p = -pi
Then we have:
f(x) = 3*cos(kx - pi)
And the period is equal to 4pi, then we must have that:
k*(x + 4pi) - pi = k*x - pi + 2pi
k = 1/2
Then the function is:
f(x) = 3*cos(x/2 - pi).
If you want to learn more about cosine functions:
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