http://slideplayer.com/3854794/13/images/18/Let+us+continue+this+process+by+adding+a+sine+wave+of+frequency+5f%2C+as+shown+in+Figure+3.7a%2C+and+then+adding+a+sine+wave+of+frequency+7f%2C+as+shown+in+Figure+3.7b.+As+we+add+additional+odd+multiples+of+f%2C+suitably+scaled%2C+the+resulting+waveform+approaches+that+of+a+square+wave+more+and+more+closely..jpg