n 
waveform into the frequency domain (a Fourier Transform), and that timbre is at least partly a matter 
of the mix of frequencies present (the spectrum), our main interest in the FM formula is in the 
spectrum it produces.  To determine that spectrum, we have to endure some tedious mathematics.  
By the trigonometric identity:
</p>

<!-- LATEX \[  \cos (a+b)=\cos a\cos b - \sin a \sin b  \] -->

<img class="indented" src="pix/fmeq5.png" alt="cos a+b">


<!-- LATEX \[B\sin \omega_{m}t  \] -->

<p>we can su