Fourier series approximation: DC component and fundamental frequency
Plotting the DC component and only the fundamental frequency means, you should plot the harmonic approximation that uses only $a_0, a_1, b_1$ (i.e. the DC component plus sines and cosines of the first frequency (which is the fundamental frequency). For the second point, you should plot the reconstruction with $a_0, a_1, b_1, a_2, b_2$. 

Exactly this problem is treated, including code, at my site.