Why a constant function doesn't have an inverse function?

When graphing functions, an inverse function will be symmetric to the original function about the line y = x. Since a constant function is simply a straight, horizontal line, its inverse would be a straight, vertical line. However, a vertical line is not a function. Therefore, constant functions do not have inverse functions.

Another way of figuring this question can be achieved using the horizontal line test. Look at your original function on a graph. If any horizontal line intersects the graph of the original function more than once, the original function does not have an inverse. The constant function is a horizontal line. Under the assumptions of the horizontal line test, a horizontal line infinitely will cross the original function. Thus, the constant function does not have an inverse function.