Question

Two waves are superposed. one wave has an amplitude of 5 cm, and other has an amplitude of 4 cm. What is the resultant amplitude?

Answer 1

If both waves have the same wavelength, then the amplitude of
their sum could be anything between 1 cm and 9 cm, depending
on the phase angle between them.

If the waves have different wavelengths, then the resultant is a beat
with an amplitude of 9 cm.