Update header.tex

Author: khanh101

assets/header.tex

@@ -75,6 +75,8 @@
 \DeclareMathOperator{\Ev}{Ev}
 \DeclareMathOperator{\Nat}{Nat}
 \DeclareMathOperator{\id}{id}
+\DeclareMathOperator{\Var}{Var}
+\DeclareMathOperator{\Cov}{Cov}
 
 %% pair delimiter
 \DeclarePairedDelimiter{\abs}{\lvert}{\rvert}