R = 1/sum(1./y);
y is a vector. You want to divide EACH of the elements of y into 1. So you use the ./ operator, which is designed to solve element-wise operations. (Just like the .* and .^ operators.)
The other divide is a scalar/scalar operation, so / works fine there, although ./ would also be acceptable. That is:
R = 1./sum(1./y);
would also work.
Finally, there was no need to put a parens around y in your original line. So while it is fine to add additional parens if you are not sure you need them in some spot, too many parens that need not be there will only make things confusing, hard to read and debug.
