Final answer:
When two coherent waves with equal amplitudes x interact in phase, the resultant wave has an amplitude of 2x. If the phase difference between the coherent waves is changed to 60 degrees, the new amplitude of the resultant wave will still be 2x.
Step-by-step explanation:
When two coherent waves with equal amplitudes x interact in phase, the resultant wave has an amplitude of 2x. This is because the waves add up constructively and their amplitudes combine. If the phase difference between the coherent waves is changed to 60 degrees, the new amplitude of the resultant wave will still be 2x. The change in phase difference does not affect the amplitude of the resultant wave.