Linearity
If a linear system has a response to exp(2jt) of t*exp(-2jt) and a response to exp(-2jt) of t*exp(2jt), then it's response to cos(2t) must be t*cos(2t).
To look at this in more detail, we can consider the following expression: $ cos(2t) = \frac{1}{2}(e^{2jt}+e^{-2jt})\! $
