\begin{align}
x(t) &=\cos...
\begin{align}
x(t) &=\cos(2\pi a t)\cos(2\pi b t)\cos(2\pi c t)
\notag\\
&={1}/{2}\cos(2\pi c t)\left\{%
\cos[2\pi (a+b) t]+\cos[2\pi (a-b) t]
\right\}%
\notag\\
&={1}/{4}\left\{%
\cos[2\pi (a+b+c) t]+\cos[2\pi (a+b-c) t\right]
\notag\\
&\ \ +\cos[2\pi (a-b+c) t\left]+\cos[2\pi (a-b-c) t]
\right\}%
.
\end{align}
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