Question 1181064
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I will use the cosine function.


The middle level is P = 0.15, and the amplitude is 0.05, obviously.


So, I write  P(t) = 0.15 + 0.05*cos(2pi*x).


The starting value for the factor  "x"  is 0  at  t= 5  ( x  must be 0 at May ), and the period is  12  months (one year).


So, I take  x = {{{(t-5)/12}}},  and then my final formula is


    P(t) = {{{0.15 + 0.05*cos(2pi*((t-5)/12))}}},      <U>ANSWER</U>


where t is the index showing the month of the year.  For clarity,  t = 5 for May.
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Solved, answered and carefully explained.