Algebra 2?

The daily level of the tide at a given harbour.
This is the time required for the Earth to rotate once relative to the Moon.
The period is usually about 12 hours and 25.2 minutes, (half a tidal lunar day)

see this WIKI link for more detail
.https://en.wikipedia.org/wiki/Tide#Principal_lunar_semi-diurnal_constituent

Periodic function is a function that repeats its values in regular intervals. The intervals are called the periods. For example all trigonometry functions are periodic like the sine function with period of 2π so we can write:
sin(x) = sin(x + 2π)
therefore a general form of periodic function is:
f(x + p) = f(x) . p ≠ 0 , where p is the period

Real world's examples of periodic functions are: Temperature during the day, tides, seasons etc.

I hope this helps.

Your question has been answered. .... I guess right