Hi Members,

Var $\left(\displaystyle\sum_{i=1}^n X_i\right) =Cov \left(\displaystyle\sum_{i=1}^n X_i,\displaystyle\sum_{j=1}^n X_j\right)$

=$\displaystyle\sum_{i=1}^n \displaystyle\sum_{j=1}^n Cov(X_i,X_j)$

=$\displaystyle\sum_{i=1}^n Cov(X_i,X_j) +\displaystyle\sum_{i}\displaystyle\sum_{j\not=i}C ov(X_i,X_j)$

=$\displaystyle\sum_{i=1}^n Var(X_i) + 2\displaystyle\sum_{i}\displaystyle\sum_{j<1} Cov(X_i,X_j)$

Now, in the last step, from where 2 arise? Can any member suggest me.