or example, that term will produce
not just (sin a)^n and (sin b)^n, but all sorts of stuff involving sin a * sin b (in various powers),
and this produces things like cos(a+b) and cos(a-b).
For a less impressionistic derivation of the spectrum, see Le Brun,
"A Derivation of the Spectrum of FM with a Complex Modulating Wave".  The result can be 
expressed:
</p>

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     \sin(\omega_{c}t + B_{1}\sin \omega_{m1}t + B_{2}\sin \omega_{m2}t) = \sum_{i=-\infty}^{\infty} \sum_{k=-\infty}^{\infty} J_