Depending on the meaning of the question, X 1*X2= 1 or-1.

If the formula is established, there should be even terms on the left side of the formula.

Then n should be a multiple of 2.

When n is an odd multiple of 2, the number of terms of X is odd. When the formula on the left is multiplied by 2 and the others are added by 0, the remaining two terms are the same.

Then n should be an even multiple of 2. That is, a multiple of 4.