Suppose that a weight on a spring has initial position s(0) and period p. s(0)= 1in ; p=0.8

The question is to find a s function given by s(t) = a cos(ωt) that models the displacement of the weight.

The function is given as:
s(t) = a cos(ωt)
This means that to get the exact function modelling the given situation, we will have to get values for ω and a.

1- getting the value of ω:
period = 0.8 sec
frequency = 1/period = 1/0.8 = 5/4 hz
ω = 2πf = 2*π*5/4 = 2.5π rad/sec
The function now became:
s(t) = a cos(2.5πt)

2- getting the value of a:
We are given that:
s(0) = 1 in
This means that:
s(t) = a cos(2.5πt)
1 = a cos(2.5π*0)
1 = a cos(0)
a = 1

Therefore, the function is:
s(t) = cos(2.5πt)

To get the value of the function at any instance (t), substitute with the value of "t" in the above function and get the corresponding s(t)

Hope this helps :)