s(-x) = \cos(x) -->

<p>Our first reaction is, "well so what if one's a sine and the other's a cosine &mdash; they'll sound the 
same", but we are being hasty.  What if (for example), the modulator has the same frequency as the 
carrier, and its index (B) is high enough that some significant energy appears at
<img src="pix/fmeq24.png" alt="w-m=-w">?
Where does energy at a negative frequency go?  We once again fall back on
trigonometry: <img src="pix/fmeq52.png" alt="sin(-x)=-sin(x)">,
but <img src="pix/fmeq