Adjective to suggest X's being either an "additive" or "multiplicative" function of A and B?

Solution 1:

This is a very surgical sentence you're trying to construct. To help cipher it out I used:

• X = a pie
• i = calories
• A = pears
• B = berries

Thus it seems you want to convey: "The total amount of calories in a pie is always some ______ function of the calories in pears and the calories in berries."

To your questions:

1. The adjective I suggest is "compounded" suggesting either additive or multiplicative properties which would mean X is never less than either A or B, individually.
2. I don't think "arithmetic" works. It's too broad and doesn't cover your second condition (X is never less than either A or B, individually).
3. I don't think I could construct a tighter sentence than "The total amount of calories in a pie is always some compounded function of the calories in pears and the calories in berries."