Precalculus (6th Edition) Blitzer

a. $y= cos(x)$ b. See graph.
a. Using the Difference Formula, we have $y=cos(x-2\pi)=cos(x)cos(2\pi)+sin(x)sin(2\pi)=cos(x)$ b. We can graph the new function $y=cos(x)$ and compare with the given graph to see that they are identical.