There are several problems here. The main one is that you are using matrix operations instead of element-wise division. In this case it doesn't matter much (as the divisor is a scalar), but it is a good habit to use ./ and .* when dealing with arrays.
The other issue is that several of the variables in your code are not defined, so we don't know what size they have. It looks like you mean X=xx(2:N) instead of X=x(2:N).
