Find a cosine function whose period is 7.

To find a cosine function with period 7, we use the formula:
Period = 2π / B

We need the period to be 7, so:
7 = 2π / B

Solving for B:
B = 2π / 7

So, the cosine function is:
y = cos(2πx / 7)

This function repeats its values every 7 units.

Cosine functions are used in waves, sound, and circular motion. The period tells us how long it takes for the function to complete one full cycle.

Thus, the required cosine function is y = cos(2πx / 7).