Do you always know the position of the terms you want to extract? Presumably, you have a vector in MATLAB and you are summing the elements of that vector, so you can just do a partial sum by using those indices.
x = [1 2 3 4 5];
sum(x(2:3))
If it's a very big vector and you're not sure the location of those elements, you can use logical indexing.
sum(x(x>1 & x<4))
The above is simply saying, look inside x for all elements greater than 1 and less than 4 (and sum those)
Or is it more complicated than that?
