Basicly the title. I know it's easy but I can't figure it out. [Edit] Not a duplicate has a 2x instead of x.
As x approaches 0 Cos(x) approaches 1 so we can in a sense think of 1/x. As x goes to 0 from the positive side 1/x approaches infinity. But when x goes to 0 from the negative side 1/x goes instead to negative infinity. This means that the limit as x goes to 0 for Cos(x)/x is undefined as the left and right limits do not agree.