What is the maximum condition; minimum condition?

The amplitude of oscillations at a given point in space is maximum if the difference in wave paths is an integer number of wavelengths.
d = kl, where k = 0; 1; 2 …
The amplitude of oscillations at a given point in space is minimal if the difference in wave paths is an odd number of half-waves.
d = (2k + 1) l / 2, where k = 0; 1; 2 …

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