(5-5) The picture below is based on fig. V-iii. The equation of each of the individual sinusoidal curves in the lower part of the picture is y = cos(2πx/λ), which we can write more concisely as y = cos(kx), where k is the angular wavenumber, 2π/λ.
The superposition at the top is the scaled sum of the five functions shown below:
y = (cos(0x) + cos(0.5x) + cos(x) + cos(1.5x) + cos(2x))/ 5.
See if you can deduce the values of x where constructive interference will generate a peak in the superposed waveform, and relate these x values to the wavelength.