Final answer:
When two waves with equal amplitudes and opposite signs interact, they produce pure destructive interference, resulting in a zero amplitude where the waves completely cancel each other out.
Step-by-step explanation:
If two waves with equal amplitudes and opposite signs interact, the result is pure destructive interference. This occurs when two identical waves arrive exactly out of phase, meaning they are precisely aligned crest to trough, as observed in figures referenced in your question. The superposition principle states that the resulting amplitude of two waves interfering will be the algebraic sum of the individual waves, leading to a case where they completely cancel each other out, resulting in an amplitude of zero for pure destructive interference.