Answer
Waves of the same frequency can interfere either completely constructively or completely destructively, as shown in Figure 20.17. The type of interference depends on whether the waves are in phase, or out of phase.
However, for 2 waves to produce beats, they must *alternate* between constructive and destructive interference at a point in space. The only way this can happen is if they have different frequencies. This is shown in Figure 20.21.