Answer
This is a hands-on demonstration.
Work Step by Step
The process is shown in Figure 21.6. When the fundamental is added to the second and third harmonics, the result resembles the wave shown in the fourth line next to the label "composite wave".
Here is a source for a diagram similar to that shown in Figure 21.6: Wikipedia contributors, "Fourier Series," Wikipedia, The Free Encyclopedia, wikipedia.org/wiki/Fourier_series (accessed March 31, 2016).
Searching the internet for images illustrating the term "Fourier decomposition" will provide you with many more examples similar to this one.
