the parseval relation as stated in (5.100) of taylor applies to a function whose fourier series happens to contain only cosines. write down the relation and prove it for a function x=re(z), subject to a force f-reg), where z(t)-σ-- cneina t and g (t) = σα--o fneinat. as we've learned, this function allows for both sines and cosines eq. (5.100) is provided as reference here