Remember that the domain is all possible x values of a function. In the case of sine and cosine functions, they go up and down (like waves), but go on forever. Therefore, the domain is all real numbers.
For the equations y = sin(x) and y = cos(x), we can replace x with any real number we want. The output y value will be between -1 and 1, inclusive of both endpoints.
The graph for each function shows a bouncing wave stretching forever to the left and right. This visually confirms the domain.
