1
edit
:<math>\textrm{Var}\left(m^{\star}\right)=\textrm{Var}\left(m\right) + c^2\,\textrm{Var}\left(t\right) + 2c\,\textrm{Cov}\left(m,t\right);</math>
It can be shown that choosing the optimal coefficient
:<math>c^\star =  \frac{\textrm{Cov}\left(m,t\right)}{\textrm{Var}\left(t\right)}; </math>

edit