We know that f (x) = cos x is a periodic function with period 2π.

∴ ψ (x) = cos (3x) is a periodic function with period 2π/3. So, we will draw the graph of ψ (x) = cos (3x) in the interval [0, 2π/3]. The values of ψ (x) = cos (3x) at various points in [0, 2π/3] are listed in the following table:

By plotting the above points, we obtain the required curve.