Trace map from $\Bbb Q\Bbb(\zeta_r)$ to $\Bbb Q\Bbb.$

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I am interested in knowing how to get values of $Tr_r(a)$.

Where $Tr_r$ is trace map from $\Bbb Q\Bbb(\zeta_r)$ to $\mathbb Q\mathbb,$ and $\zeta_r$ are some specific complex roots of unit.

Not really familiar to what a "trace map" is.