Depending on what you wish to do, it could be complex.

You would also have to design it for incremental learning levels. So, teaching a person long division would be done at one level, and set of users, while integration would be taught to a whole different set of users, with different skills.

My biggest use would be reference material. So, rather than reading the site from start to finish, I would be more likely to look up trigonometric integrals, with perhaps tables of different common integrals.

Or, perhaps looking up eigenvalues, eigenvectors, or determinants, and some kind of an explanation of what I would ever need them for. I suppose for solving sets of linear equations, but the brute force method always seems to work for me.

Or, perhaps looking up long forgotten formulas.