If you are able to write any signal then when you frequency analysis of it it generally becomes a sinusoid (fourier transform)I agree with vinodstanur.
another subject, ( as per vinodstanur status line):
I disagree that all waveforms are composed of different sinusoids. I think that although any waveform can be mathematically described by fourier series, they are generated by physical and electrical phenomenon that usually not involve sine waves. As the one which are studying here. Applying a mathematical abstraction not necessarily means that the real wolrd behaves the same way. But who knows what really happens ? We only have models.