A sinusoid $\displaystyle e^{\sigma \cdot t} \cdot \cos \omega t$ can be expressed as a sum of exponentials e^(st) and e^(-st) with complex frequencies s=sigma + jw and s=sigma jw. Locate in the complex plane the frequencies of the following sinusoids:

A) cos 4t
B) e^(-2t) * cos4t
C) 4

Putting these equations through the Euler formula is simple, but I'm not sure I understand what the question is asking. For example, take (A). I can do this:

$\displaystyle \cos (4t) = \frac{1}{2}(e^{j4t} + e^{-j4t})$

How do I answer the original question?