wave s 125 picture
dineshbabumm,
"Google" on Fourier series. You'll find a wealth of material. As far as considering odd harmonics only is concerned, consider the following: Odd harmonics have symmetry with respect to the fundamental, i.e., harmonic peaks that occur with respect to the positive half-cycle of the fundamental, have a corresponding negative peak with respect to the negative half cycle of the fundamental. (See the pictures from the replies of the other respondents). The conclusion is that any symmetrical waveform consists only of odd harmonics. A Fourier analysis of any symmetrical waveform will confirm this.
Regards,
Kral