Question:

Suppose we have three item scores, denoted as X1,X2 and X3, X1,X2 and X3, and the total score is Y=X1+X2+X2.

How to prove the following equation:Var(Y)=Var(x1)+Var (x2)+var (x3)=2Cov (X1,X2)=2Cov (X2,X3)+2Cov (X3,X1)

Thanks!