Show that the cross-product terms formed from (i· ··), (·j ··), and (ij i· ·j + X··) sum to zero, i = 1, 2, .

Show that the cross-product terms formed from (i· ˆ’ ··), (·j ˆ’ ··), and (ij ˆ’ i· ˆ’ ·j + X··) sum to zero, i = 1, 2, . . . a and j = 1, 2, . . . , b. For example, write

And sum each term in the inner summation, as grouped here, to get zero.

i=1 j=1 j-1