If two waves of equal amplitude A produce a resultant wave of the same amplitude A, it implies that they have a phase difference of 180 degrees or π radians. In other words, they are completely out of phase with each other.
When two waves interfere, their superposition determines the resulting wave. If two waves are in-phase (with a phase difference of 0 degrees or 0 radians), their amplitudes add up, resulting in a wave with a larger amplitude. Conversely, if two waves are completely out of phase, their amplitudes cancel each other out, resulting in a wave with zero amplitude.
In this case, since the resultant wave has the same amplitude as the individual waves, it indicates that their amplitudes add up constructively despite being out of phase. This can occur when two waves are perfectly 180 degrees out of phase, leading to destructive interference in terms of amplitude but constructive interference in terms of the resultant amplitude.